Add an ubifs mount helper

Add an ubifs mount helper

This abstracts away attaching of the right ubi and then selecting the right
ubi device and volume to mount.

As described in the comment at the top this allows to mount ubifs volumes
directly from /etc/fstab without having to use hardcoded numbers (which
depend on mount order and so are unreliable) and extra magic to care for
attaching.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>