UBIFS: allow sync option in rootflags

When passing UBIFS parameters via kernel command line, the
sync option will be passed to UBIFS as a string, not as an
MS_SYNCHRONOUS flag. Teach UBIFS interpreting this flag.

Reported-by: Aurélien GÉRÔME <ag@debian.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>