On a cleanly wiped device, storage must be unlocked in order to create
and retrieve the serial number. However, storage unlocking happens in
boot.py, which is imported _after_ usb.py
We must therefore sidestep asking for the serial number.
This avoids problems with large timeouts causing the scheduler queue to
think the time counter has overflown, and ordering the autolock task before
immediate tasks.
The maximum reasonable time difference is 0x20000000, which in
microseconds is ~8 minutes, but in milliseconds a more reasonable ~6
days.
expose ff.c constants, raise them as arguments to FatFSError
introduce NotMounted and NoFilesystem as subclasses of FatFSError with
the appropriate error code set
Instead of having possibly multiple FatFS objects, each with its own
`fs` struct, there is one global static fs_instance. This is to match
the mode of operation of ff.c, which assumes a global list of mounts,
and all functions operate on the global based on path.
Methods of FatFS were converted to functions on the fatfs module.
fatfs.unmount() does not call ff.c's unmount, but simply invalidates
fs_instance. This is basically what ff.c would do, except without
messing with ff.c's global list of mounts.
