fs/ubifs: spin-off ubi to be its own filesystem

fs/ubifs: spin-off ubi to be its own filesystem

Currently, the ubifs-in-ubi-image ("UBI image" thereafter) filesystem
is half an option of the ubifs filesystem, half a filesystem on its
own: the config options are options of the ubifs filesystem, but the
.mk code is in a separate .mk and registers a real filesystem.

Make it a full filesystem on its own, in its own directory tree.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>