This simplifies configuration by removing the `MICROPY_PY_OS_SEP` option
and instead including `os.sep` if `MICROPY_VFS` is enabled. That matches
the configuration of all existing ports that enabled `os.sep` (they also
had `MICROPY_VFS` enabled), and brings consistency to other ports.
Fixes issue #15116.
Signed-off-by: Damien George <damien@micropython.org>
8159dcc276