Flash size as seen by vendor SDK doesn't depend on real size, but rather on
a particular value in firmware header, as put there by flash tool. That means
it's user responsibility to know what flash size a particular device has, and
specify correct parameters during flashing. That's not end user friendly
however, so we try to make it "flash and play" by detecting real size vs
from-header size mismatch, and correct the header accordingly.
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.
