This commit changes the esp8266 boards to use littlefs v2 as the
filesystem, rather than FAT.  Since the esp8266 doesn't expose the
filesystem to the PC over USB there's no strong reason to keep it as FAT.
Littlefs is smaller in code size, is more efficient in use of flash to
store data, is resilient over power failure, and using it saves about 4k of
heap RAM, which can now be used for other things.

This is a backwards incompatible change because all existing esp8266 boards
will need to update their filesystem after installing new firmware (eg
backup old files, install firmware, restore files to new filesystem).

As part of this commit the memory layout of the default board (GENERIC) has
changed.  It now allocates all 1M of memory-mapped flash to the firmware,
so the filesystem area starts at the 2M point.  This is done to allow more
frozen bytecode to be stored in the 1M of memory-mapped flash.  This
requires an esp8266 module with 2M or more of flash to work, so a new board
called GENERIC_1M is added which has the old memory-mapping (but still
changed to use littlefs for the filesystem).

In summary there are now 3 esp8266 board definitions:
- GENERIC_512K: for 512k modules, doesn't have a filesystem.
- GENERIC_1M: for 1M modules, 572k for firmware+frozen code, 396k for
  filesystem (littlefs).
- GENERIC: for 2M (or greater) modules, 968k for firmware+frozen code,
  1M+ for filesystem (littlefs), FAT driver also included in firmware for
  use on, eg, external SD cards.

Using new options enabled in the uncrustify configuration.

Only included in GENERIC build.

This function is not used by the core but having it as part of the build
allows it to be used by user C modules, or board extensions.  The linker
won't include it in the final firmware if it remains unused.

Fixes issue #5780.

Using new options enabled in the uncrustify configuration.

Fixes issue #5764.

Otherwise ets_loop_iter may be reentered.  Related to issue #5714.

This is run with uncrustify 0.70.1, and black 19.10b0.

This string is recognised by uncrustify, to disable formatting in the
region marked by these comments.  This is necessary in the qstrdef*.h files
to prevent modification of the strings within the Q(...).  In other places
it is used to prevent excessive reformatting that would make the code less
readable.

This commit adds mp_raise_msg_varg(type, fmt, ...) as a helper for
nlr_raise(mp_obj_new_exception_msg_varg(type, fmt, ...)).  It makes the
C-level API for raising exceptions more consistent, and reduces code size
on most ports:

   bare-arm:   +28 +0.042%
minimal x86:  +100 +0.067%
   unix x64:   -56 -0.011%
unix nanbox:  -300 -0.068%
      stm32:  -204 -0.054% PYBV10
     cc3200:    +0 +0.000%
    esp8266:   -64 -0.010% GENERIC
      esp32:  -104 -0.007% GENERIC
        nrf:  -136 -0.094% pca10040
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS

It was originally in IRAM due to the linker script specification, but
since the function moved from lib/utils/interrupt_char.c to py/scheduler.c
it needs to be put back in IRAM.

Previous behaviour is when this argument is set to "true", in which case
the function will raise any pending exception.  Setting it to "false" will
cancel any pending exception.

The ability to change the host is a frequently requested feature, so
explicitly document how it can be achieved using the existing code.

See issues #2121, #4385, #4622, #5122, #5536.

Prior to this commit, if the flash filesystem was not formatted then it
would error: "AttributeError: 'FlashBdev' object has no attribute 'mount'".
That is due to it not being able to detect the filesystem on the block
device and just trying to mount the block device directly.

This commit fixes the issue by just catching all exceptions.  Also it's not
needed to try the mount if `flashbdev.bdev` is None.

Many ports already allow overriding CROSS_COMPILE.  This modifies the
remaining ports to allow it as well.

Move webrepl support code from ports/esp8266/modules into extmod/webrepl
(to be alongside extmod/modwebrepl.c), and use frozen manifests to include
it in the build on esp8266 and esp32.

A small modification is made to webrepl.py to make it work on non-ESP
ports, i.e. don't call dupterm_notify if not available.

Implements text, rodata and bss generalised relocations, as well as generic
qstr-object linking.  This allows importing dynamic native modules on all
supported architectures in a unified way.

This commit removes the Makefile-level MICROPY_FATFS config and moves the
MICROPY_VFS_FAT config to the Makefile level to replace it.  It also moves
the include of the oofatfs source files in the build from each port to a
central place in extmod/extmod.mk.

For a port to enabled VFS FAT support it should now set MICROPY_VFS_FAT=1
at the level of the Makefile.  This will include the relevant oofatfs files
in the build and set MICROPY_VFS_FAT=1 at the C (preprocessor) level.

With this commit an esp8266-based board can now be built with littlefs
support via, eg "make MICROPY_VFS_LFS2=1".

This helper function was added a while ago and these are the remaining
cases to convert, to save a bit of code size.

Also rename SEC_COUNT to BLOCK_COUNT and SEC_SIZE to BLOCK_SIZE.

This commit implements automatic module weak links for all built-in
modules, by searching for "ufoo" in the built-in module list if "foo"
cannot be found.  This means that all modules named "ufoo" are always
available as "foo".  Also, a port can no longer add any other weak links,
which makes strict the definition of a weak link.

It saves some code size (about 100-200 bytes) on ports that previously had
lots of weak links.

Some changes from the previous behaviour:
- It doesn't intern the non-u module names (eg "foo" is not interned),
  which saves code size, but will mean that "import foo" creates a new qstr
  (namely "foo") in RAM (unless the importing module is frozen).
- help('modules') no longer lists non-u module names, only the u-variants;
  this reduces duplication in the help listing.

Weak links are effectively the same as having a set of symbolic links on
the filesystem that is searched last.  So an "import foo" will search
built-in modules first, then all paths in sys.path, then weak links last,
importing "ufoo" if it exists.  Thus a file called "foo.py" somewhere in
sys.path will still have precedence over the weak link of "foo" to "ufoo".

See issues: #1740, #4449, #5229, #5241.

A release also sets: UART_OS = -1

When loading a manifest file, e.g. by include(), it will chdir first to the
directory of that manifest.  This means that all file operations within a
manifest are relative to that manifest's location.

As a consequence of this, additional environment variables are needed to
find absolute paths, so the following are added: $(MPY_LIB_DIR),
$(PORT_DIR), $(BOARD_DIR).  And rename $(MPY) to $(MPY_DIR) to be
consistent.

Existing manifests are updated to match.

Removes symlinks in modules directory, all frozen code is now specified by
manifest.py.

Fixes issue #5189.

The specific board can be selected with the BOARD makefile variable.  This
defaults (if not specified) to BOARD=GENERIC, which is the original default
firmware build.  For the 512k target use BOARD=GENERIC_512K.

Via the standard tx/rx arguments: UART(0, 115200, tx=Pin(15), rx=Pin(13)).

Resolves issue #4718.

Since the esp8266 has a small stack this buffer is kept in the BSS.

So that the timeout for machine.time_pulse_us() can be large.

Fixes issue #2775.