All variants now use extmod/moduos.c as their uos module implementation.
In particular this means they all have MICROPY_VFS enabled and use VfsPosix
for their filesystem.
As part of this, the available functions in uos become more consistent with
other ports:
- coverage variant gets uos.urandom
- minimal and standard variant get: unlink, chdir, getcwd, listdir
Signed-off-by: Damien George <damien@micropython.org>
