"" is the correct name of the root directory when mounting a device there
(as opposed to "/"). One can now do os.listdir('/') and open('/abc'), as
well as os.listdir() and open('abc').
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.
