This commit is a combination of 216 commits from the initial stages of
development of this port, up to and including the point where the code was
moved to the ports/esp32 directory.  These commits were mostly concerned
with setting up the build system and getting a reliable port working with
basic features.  The following is a digest of the original commits in their
original order (most recent listed first), grouped where possible per
author.  The list is here to give credit for the work and provide some
level of traceability and accountability.  For the full history of
development please consult the original repository.

All code is MIT licensed and the relevant copyright holders are listed in
the comment-header of each file.

Damien George <damien.p.george@gmail.com>
  esp32: Update to latest ESP IDF.
  esp32: Update module symlinks now that code lives under the ports dir.
  esp32: Update to compile with new ports/esp32 directory structure.
  esp32: Move it to the ports/ directory.
  esp32/machine_uart: Don't save baudrate but compute it instead.
  esp32/modsocket: Add socket.readinto() method.
  esp32/modesp: Add esp.gpio_matrix_in and esp.gpio_matrix_out functions.
  esp32/machine_uart: Wait for all data to be tx'd before changing config.

NyxCode <moritz.bischof1@gmail.com>
  esp32: Add note to README.md about updating the submodules of ESP IDF.

Anthony Briggs <anthony.briggs@gmail.com>
  esp32: Update README.md installation and flashing instructions.

Javier Candeira <javier@candeira.com>
  esp32: Raise error when setting input-only pin to output.

  With help from Adrian Smith (fon@thefon.net)

Javier Candeira <javier@candeira.com>
  esp32: Replace exception raising with corresponding mp_raise_XXX funcs.

Tisham Dhar <whatnickd@gmail.com>
  esp32: Add some specific notes about building on Windows using WSL.

Ben Gamari <ben@smart-cactus.org>
  esp32: Provide machine.Signal class.

Damien George <damien.p.george@gmail.com>
  esp32/modnetwork: Implement AP version of network.isconnected().

Eric Poulsen <eric@zyxod.com>
  esp32/README.md: Add note about btree submodule initialization.

Damien George <damien.p.george@gmail.com>
  esp32: Make firmware.bin start at 0x1000 to allow flash size autodetect.
  esp32: Changes to follow latest version of upstream uPy.
  esp32/Makefile: Separate ESP-specific inc dirs to avoid header clashes.
  esp32: Enable "btree" database module.
  esp32: Update to latest ESP IDF.

Roosted7 <thomasroos@live.nl>
  esp32: Update to latest ESP-IDF.

Alex King <alex_w_king@yahoo.com>
  esp32/machine_timer: Add support for esp32 hardware timer.

  Code lineage:
  Timer() is based loosely on the version in esp8266, although the
  implementation is differs significantly because of the change in
  the underlying platform.

Damien George <damien.p.george@gmail.com>
  esp32/machine_uart: Increase UART TX buffer size to 64.
  esp32/modules: Update dht symlink.
  esp32/mpconfigport.h: Enable utimeq module, needed for uasyncio.
  esp32: Changes to follow latest version of upstream uPy.
  esp32: Update to latest ESP-IDF.
  esp32/machine_uart: Add uart.any() method.
  esp32/machine_uart: Uninstall the UART driver before installing it.

Thomas Roos <mail@thomasroos.nl>
  esp32: Update to latest ESP-IDF.

Eric Poulsen <eric@zyxod.com>
  esp32/modsocket: Make read/write return None when in non-blocking mode.
  esp32/modsocket.c: Fix send/sendto/write for non-blocking sockets.

Odd Stråbø <oddstr13@openshell.no>
  esp32: Initial working implementation of machine.UART.

  Code lineage (as seen by previous commits): I copied the ESP8266 code,
  renamed pyb -> machine, and used esp-idf as a reference while implementing
  minimal functionality.  I provide all of my changes under the MIT license.

Odd Stråbø <oddstr13@openshell.no>
  esp32/machine_uart: Rename pyb to machine.
  esp32: Copy machine_uart.c from esp8266 port.

Damien George <damien.p.george@gmail.com>
  esp32/moduos: Add uos.ilistdir() function.
  esp32: Mount filesystem at the root of the VFS, following esp8266.

Andy Valencia <vandyswa@gmail.com>
  esp32: Add hardware SHA1/SHA256 support via mbedtls API.

  Code lineage: a copy of extmod/moduhashlib with the API invocation details
  edited.  Total derivative work.

Andy Valencia <vandyswa@gmail.com>
  esp32: Add PWM support via machine.PWM class.

  Code lineage:
  I started by copying the esp8266 machine_pwm.c. I used information from the
  ESP32 Technical Reference Manual, the esp-idf documentation, and the SDK's
  sample ledc example code (but I did not copy that code, just studied it to
  understand the SDK's API for PWM). So aside from the code copied from the
  esp8266 PWM support, everything else you see is just new code I wrote.

  I wasn't an employee of anybody when I wrote it, and I wrote it with the
  understanding and intention that it's simply a derivative work of the
  existing micropython code. I freely and willingly contribute it to the
  project and intend that it not change the legal status of the micropython
  code base in any way, even if it is included in that base in whole or part.

Damien George <damien.p.george@gmail.com>
  esp32/modules: Add symlinks for upysh and upip.

Eric Poulsen <eric@zyxod.com>
  esp32/modmachine: Add unique_id() function to machine module.

Damien George <damien.p.george@gmail.com>
  esp32: Change dac_out_voltage to dac_output_voltage for new IDF API.
  esp32: Update esp32.custom_common.ld to align with changes in ESP IDF.

Eric Poulsen <eric@zyxod.com>
  esp32: Update to latest ESP IDF.

Damien George <damien.p.george@gmail.com>
  esp32/modsocket: When resolving IP addr handle the case of host=''.
  esp32: Update to latest ESP IDF.

Eric Poulsen <eric@zyxod.com>
  esp32/Makefile: Change default FLASH_MODE to dio for WROOM-32 module.
  esp32: Move FAT FS to start at 0x200000 and increase size to 2MiB.

Damien George <damien.p.george@gmail.com>
  esp32: Remove enable_irq/disable_irq and use ATOMIC_SECTION instead.
  esp32/mpconfigport.h: Provide ATOMIC_SECTION macros.
  esp32/main: Restart the MCU if there is a failed NLR jump.

Daniel Campora <daniel@pycom.io>
  esp32: Enable threading; be sure to exit GIL when a thread will block.
  esp32: Trace the registers when doing a gc collect. Also make it thread ready.
  esp32: Add threading implementation, disabled for the time being.

Damien George <damien.p.george@gmail.com>
  esp32: Update to latest ESP IDF.
  esp32/uart: Use high-level function to install UART0 RX ISR handler.
  esp32/Makefile: Make FreeRTOS private include dir really private.

Eric Poulsen <eric@zyxod.com>
  esp32: Add support for hardware SPI peripheral (block 1 and 2).

Sergio Conde Gómez <skgsergio@gmail.com>
  esp32/modules/inisetup.py: Mount filesystem at /flash like ESP8266

Damien George <damien.p.george@gmail.com>
  esp32: Convert to use core-provided KeyboardInterrupt exception.
  esp32: Pump the event loop while waiting for rx-chr or delay_ms.
  esp32: Implement Pin.irq() using "soft" scheduled interrupts.
  esp32: Update to latest ESP IDF version.

Eric Poulsen <eric@zyxod.com>
  esp32/README: Add troubleshooting section to the end.

tyggerjai <tyggerjai@gmail.com>
  esp32: Add support for WS2812 and APA106 RGB LEDs.

Damien George <damien.p.george@gmail.com>
  esp32: Add makeimg.py script to build full firmware; use it in Makefile.
  esp32/modsocket: Make socket.read return when socket closes.
  esp32/modsocket: Initialise the timeout on an accepted socket.
  esp32/mphalport: Provide proper implementations of disable_/enable_irq.
  esp32/modmachine: Add disable_irq/enable_irq functions.

Nick Moore <nick@zoic.org>
  esp32/modsocket.c: add comment explaining timeout behaviour
  esp32/modsocket.c: clean up send methods for retries too
  esp32/modsocket.c: sockets always nonblocking, accept timeout in modsocket
  esp32: Update to latest ESP IDF version.
  esp32/modsocket.c: remove MSG_PEEK workaround on select ioctl.
  esp32/modsocket.c: Initialize tcp when modsocket loads.

Damien George <damien.p.george@gmail.com>
  esp32/main: Bump heap size from 64k to 96k.
  esp32/modutime: Add time.time() function.
  esp32/modsocket: Convert lwip errnos to uPy ones.
  esp32/modules: Provide symlink to ds18x20 module.
  esp32: Add support for onewire protocol via OneWire module.
  esp32: Add support for DHT11 and DHT22 sensors.
  esp32/mphalport: Improve delay and ticks functions.
  esp32: Update to latest ESP IDF.
  esp32/modules: Provide symlink to urequests from micropython-lib.
  esp32: Populate sys.path.

Nick Moore <nick@zoic.org>
  esp32/machine_dac.c: implement DAC pins as well
  esp32/machine_adc.c: also machine.ADC
  esp32/machine_touchpad.c: add support for touchpad

Damien George <damien.p.george@gmail.com>
  esp32/README: Add hint about using GNUmakefile on case-insensitive FS.
  esp32/mpconfigport.h: Enable maximum speed software SPI.
  esp32: Provide improved version of mp_hal_delay_us_fast.
  esp32/mpconfigport.h: Enable MICROPY_PY_BUILTINS_POW3 option.
  esp32: Update to latest ESP IDF.
  esp32: Convert to use new oofatfs library and generic VFS sub-system.
  esp32: Enable help('modules') to list builtin modules.
  esp32: Convert to use new builtin help function.

Aaron Kelly <AaronKelly@email.com>
  esp32/README: Add comment about ESP-IDF version

Damien George <damien.p.george@gmail.com>
  esp32: Consistently use size_t instead of mp_uint_t.
  esp32: Change "Micro Python" to "MicroPython" in license comments.
  esp32/Makefile: Use -C argument to git instead of cd'ing.
  esp32/help: Add section to help about using the network module.
  esp32/README: Add section about configuring and using an ESP32 board.
  esp32/README: Remove paragraph about buggy toolchain, it's now fixed.
  esp32/modnetwork: Change network init logging from info to debug.
  esp32/modnetwork: Don't start AP automatically when init'ing wifi.
  esp32/modsocket: Implement socket.setsockopt, to support SO_REUSEADDR.
  esp32: Update to latest ESP IDF.
  esp32/sdkconfig.h: Remove unused CONFIG_ESPTOOLPY_xxx config settings.
  esp32/modsocket: Add support for DGRAM and RAW, and sendto/recvfrom.
  esp32/modsocket: Fix return value of "port" in socket.accept.
  esp32/modsocket: Make socket.recv take exactly 2 args.
  esp32: Enable ussl module, using mbedtls component from ESP IDF.
  esp32/modsocket: Rename "socket" module to "usocket".
  esp32/sdkconfig: Increase max number of open sockets from 4 to 8.
  esp32/modsocket: Add error checking for creating and closing sockets.
  esp32/modsocket: Use _r (re-entrant) versions of LWIP socket API funcs.
  esp32/modsocket: Raise an exception if socket.connect did not succeed.
  esp32/modsocket: Make socket.accept return a tuple: (client, addr).
  esp32/modsocket: Use m_new_obj_with_finaliser instead of calloc.
  esp32/Makefile: Add check for IDF version, and warn if not supported.
  esp32/esp32.custom_common.ld: Update to follow changes in IDF.
  esp32: Update to latest ESP IDF.

nubcore <x@nubcore.com>
  esp32: add #define CONFIG_ESP32_WIFI_RX_BUFFER_NUM 25

Nick Moore <nick@zoic.org>
  esp32/modsocket.c: add in sendall and makefile methods adafruit#10
  esp32/modsocket.c: fixups for adafruit#10
  esp32/modsocket.c: fix copyright, socket_recv gets param and exception
  esp32/modnetwork.c: fix copyright, network.active param to bool

Damien George <damien.p.george@gmail.com>
  esp32/modnetwork: Implement wlan.isconnected() method.
  esp32/modnetwork: Add initial implementation of wlan.config().
  esp32/modnetwork: Simplify event_handler messages.

Nick Moore <nick@zoic.org>
  esp32/modsocket.c: support for ioctl, settimeout, setblocking, getaddrinfo

Damien George <damien.p.george@gmail.com>
  esp32/README: Add comment about FLASH_MODE being dio.
  esp32: Update to latest ESP IDF.
  esp32/modnetwork: Remove unnecessary indirection variable for scan list.
  esp32/modnetwork: Check that STA is active before trying to scan.
  esp32/mphalport: Replace portTICK_RATE_MS with portTICK_PERIOD_MS.
  esp32/README: Add comment about using $(HOME) in makefile.
  esp32/modnetwork: Use memset instead of bzero, the latter is deprecated.
  esp32/modnetwork: Improve error handling when STA is connecting to AP.
  esp32/Makefile: Use tab instead of spaces, and use shorter variable.

Nick Moore <nick@zoic.org>
  esp32/modsocket.c: AF_*, SOCK_* and IPPROTO_* constants
  esp32/modsocket.c: socket.settimeout implementation

Damien George <damien.p.george@gmail.com>
  esp32/Makefile: Update to latest ESP IDF.

Nick Moore <nick@zoic.org>
  esp32/modsocket.c: use mp streams for sockets
  esp32: network.WLAN.ifconfig based on esp8266 version
  esp32: Fix up exception handling
  esp32: sketchy modsocket ... revisit this once modnetwork is sorted
  esp32: First cut at modnetwork, manually rebased from prev. version

Damien George <damien.p.george@gmail.com>
  esp32/help: Update help text.
  esp32: Add info about Microbric Pty Ltd being the sponsor of this port.
  esp32: Add README.md file.
  esp32/mpconfigport.h: Add weak links to many of the builtin modules.
  esp32: Enable soft implementation of machine.SPI class.
  esp32/Makefile: Simplify APP_LD_ARGS by using OBJ variable.
  esp32/Makefile: Reorganise Makefile and add some comments.
  esp32/Makefile: Clean up CFLAGS for ESP IDF components.
  esp32/Makefile: Tidy up names of ESP IDF components, to match dir name.
  esp32/Makefile: Define and use ESPCOMP variable.
  esp32: Update to latest ESP IDF.
  esp32/main: Enable filesystem support.
  esp32: Use custom ld script to ensure correct code get placed in iram.
  esp32: Update to latest ESP IDF.
  esp32/main: Pin the uPy task to core 0.
  esp32: Update to use latest ESP IDF.
  esp32: Disable boot-up scripts, spi_flash_erase_sector no longer works.
  esp32: Add scripts to init and mount filesystem.
  esp32: Enable frozen bytecode, with scripts stored in "modules/" dir.
  esp32/modesp: Increase flash_user_start position to 1Mbyte.
  esp32/Makefile: Add "erase" target for convenient erasure.
  esp32/sdkconfig: Reorder config settings to put common things together.
  esp32/sdkconfig: Change to use single core only.
  esp32: Add esp module.
  esp32/uart.c: Make sure uart ISR handler is all in iram.
  esp32/main.c: Use ESP_TASK_PRIO_MIN + 1 for mp_task's priority.
  esp32/Makefile: Use only bare-minimum flags when compiling .S files.
  esp32/Makefile: Rename "firmware" to "application".
  esp32: Update ESP IDF version.
  esp32/Makefile: Add declarations to build bootloader and partitions.
  esp32/Makefile: When deploying, write the application last.
  esp32/Makefile: Use $(INC) variable instead of listing include dirs.
  esp32/Makefile: Use locally built versions of freertos and newlib libs.
  esp32: Add low-level uart handler with ISR and ringbuf for stdin.
  esp32: Add machine.idle() function.
  esp32: Add machine.I2C class.
  esp32: Enable machine.time_pulse_us.
  esp32: Add initial implementation of machine.Pin class.
  esp32: Prepare main.c for using xTaskCreateStatic.
  esp32: Clean up mphalport.h.
  esp32: Add initial uos module.
  esp32: Clean up mpconfigport.h, enable more features.
  esp32: Use new reset function.
  esp32: Update to latest ESP IDF.
  esp32: Add idf-version target to Makefile, to track IDF commit.
  esp32: Initial port to ESP32.

updating my branch

It was incorrect to NULL out the pointer to our heap allocated buffer in
`reset`, because subsequent to framebuffer_reset, but while
the heap was still active, we could call `get_bufinfo` again,
leading to a fresh allocation on the heap that is about to be destroyed.

Typical stack trace:
```
#1  0x0006c368 in sharpdisplay_framebuffer_get_bufinfo
#2  0x0006ad6e in _refresh_display
#3  0x0006b168 in framebufferio_framebufferdisplay_background
#4  0x00069d22 in displayio_background
adafruit#5  0x00045496 in supervisor_background_tasks
adafruit#6  0x000446e8 in background_callback_run_all
adafruit#7  0x00045546 in supervisor_run_background_tasks_if_tick
adafruit#8  0x0005b042 in common_hal_neopixel_write
adafruit#9  0x00044c4c in clear_temp_status
adafruit#10 0x000497de in spi_flash_flush_keep_cache
adafruit#11 0x00049a66 in supervisor_external_flash_flush
adafruit#12 0x00044b22 in supervisor_flash_flush
adafruit#13 0x0004490e in filesystem_flush
adafruit#14 0x00043e18 in cleanup_after_vm
adafruit#15 0x0004414c in run_repl
adafruit#16 0x000441ce in main
```
When this happened -- which was inconsistent -- the display would keep
some heap allocation across reset which is exactly what we need to avoid.

NULLing the pointer in reconstruct follows what RGBMatrix does, and that
code is a bit more battle-tested anyway.

If I had a motivation for structuring the SharpMemory code differently,
I can no longer recall it.

Testing performed: Ran my complicated calculator program over multiple
iterations without observing signs of heap corruption.

Closes: adafruit#3473