Partition table and filesystem size

ESP32 firmware contains a bootloader, partition table and main application

firmware, which are all stored in (external) SPI flash. The user filesystem

is also stored in the same SPI flash. By default, MicroPython does not have

a fixed entry in the ESP32 partition table for the filesystem. Instead it

will automatically determine the size of the SPI flash upon boot, and then --

so long as there is no existing partition called "vfs" or "ffat" -- it will

create a partition for the filesystem called "vfs" which takes all of the

remaining flash between the end of the last defined partition up until the

end of flash.

This means that firmware built for, say, a 4MiB flash will work on any

device that has at least 4MiB flash. The user "vfs" filesystem will then

take up as much space as possible.

This auto-detection behaviour can be overridden: if the ESP32 partition

table does contain an entry called "vfs" or "ffat" then these are used for

the user filesystem and no automatic "vfs" partition is added. This is

useful in cases where only the MicroPython ESP32 application is flashed to

the device (and not the bootloader or partition table), eg when deploying

.uf2 files.