These will be needed for MSC support. The change matches the changes
for the bootloader.

The DTCM section got too large.

This attribute is present on builds with MSC support, allowing _boot.py
to decide about creating if and which file system.

As a benefit, it will not destroy an existing FAT partition.

Using the good implementation by @iabdalkader for RP2 and some of
@alphaFred's work for the bootlaoder.

Open topics:

- In the descriptor the blocksize was set to 64. MIMXRT requires 512
- Adding a lock preventing local write as long as the disk is mounted.
- Hyperflash has a sector size of 256k. Here, the
  BLOCK_SIZE == SECTOR_SIZE_BYTES is 4096, which by good luck meets the
  size required for SD Cards.

Enable MSC unless defined different for a board.
Disable MSC for MIMXRT1050_EVK because of it's hyperflash chip.

Porting PR micropython#8814 of @iabdalkader to the mimxrt platform.
- When the board is mounted to a PC, local write access is blocked.
- Local write access will be re-enabled again if the disk is ejected.
- If the board is not connected to a PC on start-up, local write access
  is enabled.
- The status of the write access can be determined by bdev.ioctl(7, 0).
  If the device is mounted to a PAC and thus local write access is
  blocked, it return 1, otherwise 0.
- low level write/erase using the block device calls are still possible.
  It is up to the user to check the device status ahead.

That way it is guaranteed, that a new file system is created before
an MSC mount attempt can be made.
Also: Check for readonly mode before trying mkfs.

Since remount after MSC eject does not update the file system's data
in memory, keep file the system readonly until after a soft reset, which
has to be done deliberately.
That's neither comfortable nor elegant, but safe.

Disabled MSC as default option.
Disabled MSC for MIMXRT1050_EVK deliberately.
Enabled MSC for MIMXRT1020_EVK, Teensy 4.1 and OLIMEX_RT1010 for testing.

To enble/disable MSC at boot time. The calls are a simplified version
if pyb.usb_mode():

mimxrt.usb_mode(modestr)

with modestr being either "vcp" or "vcp+msc".

The call has to be placed in boot.py and requires a hard reset to get
effective, since tusb_init() is executed only once after reset.

As a side effect, USB is not active in boot.py. So there is not USB
I/O possible in boot.py, and code running in boot.py cannot by stopped
with Ctrl-C.

1. USB will be enabled BEFORE boot.py is executed, avoiding deadlocks
   by user written code.
2. MSC support is enabled in _boot.py according to the following rules:
   a) For a FAT file system, MSC will be enabled by default.
   b) For a LFS file system, MSC will be disabled by default.
   c) The setting can be overriden by a file with the name
      set_usb_mode.py, which can define usb_mode with the lines:

      usb_mode = "vcp+msc"  # enable MSC

      or

      usb_mode = "vcp" # do not enable MSC

    If that file does not exist or is faulty, the default is used.

Including some notes about littlfs file system and MSC access.

When the board is formatted with littlfs, the FS access method by the
PC is different.