Should work on any machine that provides the correct pin functions.

Main entry point is _boot.py which checks whether FAT FS in flash mountable,
and if so, mounts it. Otherwise, it checks if flash is empty, and if so,
performs initial module setup: makes FAT FS, configures default AP name,
etc. As a last option, if flash is not empty, and could not be mounted,
it means filesystem corruption, and warning message with instructions is
printed in an infinite loop.

When lwIP creates a incoming connection socket of a listen socket, it
sets its recv callback to one which discards incoming data. We set
proper callback only in accept() call, when we allocate Python-level
socket where we can queue incoming data. So, in lwIP accept callback
be sure to set recv callback to one which tells lwIP to not discard
incoming data.

They call into vendor SDK functions system_get_free_heap_size() and
system_print_meminfo() respectively.

Saves 16 bytes of code.

Also, use mp_obj_get_int_truncated to allow integers as big as a machine
word to be passed as the value.

Upon start-up, _boot module is executed from frozen files to do early
initialization, e.g. create and mount the flash filesystem. Then
"boot.py" is executed if it exists in the filesystem. Finally, "main.py"
is executed if exists to allow start-on-boot user applications.

This allows a user to make a custom boot file or startup application
without recompiling the firmware, while letting to do early initialization
in Python code.

Based on RFC https://github.com/micropython/micropython/issues/1955.

Allows to set fragment type (txt/bin/etc.) for output records.

Will call underlying C virtual methods of stream interface. This isn't
intended to be added to every stream object (it's not in CPython), but
is convenient way to expose extra operation on Python side without
adding bunch of Python-level methods.

Also, handle continuation frames (untested).

This is strange asymmetry which is sometimes needed, e.g. for WebREPL: we
want to process only available input and no more; but for output, we want
to get rid of all of it, because there's no other place to buffer/store
it. This asymmetry is akin to CPython's asyncio asymmetry, where reads are
asynchronous, but writes are synchronous (asyncio doesn't expect them to
block, instead expects there to be (unlimited) buffering for any sync write
to completely immediately).

After this you need only one path for build (path/to/djgpp/bin). Original patch made by @dhylands

Addresses issue #1972.

The docs are still heavily biased towards WiPy, so will need a lot of
exclusions.

SOF irqs are now standard for rx/tx USB transfers, so enable them for both
FS and HS modes.  Fixes #1944.

Addresses issue #1965.

Because different emitters need to handle with-cleanup in different ways.