Calculate the unused flash area on the target device.  The values will be
exposed by _unused_flash_start and _unused_flash_length.  The start address
and the length are not aligned to either word or pages.

This commit adds the "nrf" module for port specific modules and objects.
Included in it is the "Flash" object which exposes a block device
implementation to access internal SoC flash.

Thanks to @aykevl aka Ayke van Laethem for the initial implementation.

Update flash.c to also be compiled in when
MICROPY_HW_ENABLE_INTERNAL_FLASH_STORAGE is enabled and SoftDevice is
present.

Update bluetooth/ble_drv.c to forward flash events to flash.c when
MICROPY_HW_ENABLE_INTERNAL_FLASH_STORAGE is enabled.

This documents parameters that can be passed to make to enable a specific
file system to included in the build.  Also, document the Makefile override
parameter "FS_SIZE" that can be used to tune the size of the flash region
to use as internal flash file system.

Disable MICROPY_FATFS_MULTI_PARTITION configuration because there is no
partition table in the flash for FATFS to read.

Also, set MICROPY_FATFS_MAX_SS to the size of a flash page.  For nrf51 the
value 1024 is set.  For nrf52/nrf91 the value 4096 is set.

Add posibility to override linker script "_fs_size" from make by adding the
FS_SIZE parameter.  The syntax of value is linker script syntax.  For
example, the value of 131072 bytes can be written as 128K like this:
FS_SIZE=128K.

If not set, default value for "_fs_size" from linker script will be used.

Enable MICROPY_PY_IO_FILEIO if MICROPY_VFS_FAT, MICROPY_VFS_LFS1
or MICROPY_VFS2 has been enabled.

Enable the following features for all boards except
nrf51 boards with SoftDevice present:

- MICROPY_VFS
- MICROPY_PY_NRF
- MICROPY_HW_ENABLE_INTERNAL_FLASH_STORAGE

Update the Makefile to handle FROZEN_MANIFEST, and the README with some
small samples on how to use freeze manifests.  And add BOARD_DIR to the
Makefile which can be referenced in boards/<board>/mpconfigboard.mk to
include a board specific manifest.

Add a helper script _mkfs.py which automatically formats the file system if
nrf.Flash() is located and a VFS file system has been included in the
compilation.

The precedence is: first LFS1, LFS2 then FAT.

Set the default manifest to "modules/manifest.py".  This includes files
from the folder "modules/scripts".  The manifest default value is overriden
by all nrf51 boards that have SoftDevice present (SD=s110) to save flash.

Also add "modules/manifest.py" which is set to freeze
"modules/scripts/_mkfs.py".

extmod/vfs_lfs.c needs to resolve `mp_hal_time_ns()` in order to calculate
a timestamp from 1970 epoch.  A wall clock is not available in the nrf
port, hence the function is implemented to resolve compilation linkage
error.  The function always return 0.

Enable LittleFS v2 for all targets, except nrf51 targets when SoftDevice is
present.

This was missed in the initial implementation of the uos module.

Signed-off-by: Damien George <damien@micropython.org>