The 512k build does not have a filesystem so there is no reason to include
the filesystem-related modules.  This commit provides a custom manifest.py
for this board which no longer includes: _boot.py, flashbdev.py,
inisetup.py, upip.py, upip_utarfile.py.  This cuts the build down by about
9k of flash.
Signed-off-by: NDamien George <damien@micropython.org>

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.

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.

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.