This PR 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 (ie backup old files, install firmware, restore files to new filesystem).

As part of this PR the memory layout of the default board 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).

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_2M for 2M (or greater) modules, 968k for firmware+frozen code, 1M+ for filesystem (littlefs)

The plan is to have all 3 available for download at https://micropython.org/download