ubifs: Add UBI image generation support.

UBI images are generated from UBIFS one (with ubinize tool) and are used by
bootloaders (eg U-Boot) to write UBIFS images directly on bare NAND FLASH
(see http://www.linux-mtd.infradead.org/faq/ubi.html

).

[Peter: tweak help text]
Signed-off-by: Julien Boibessot <julien.boibessot@armadeus.com>
Signed-off-by: Nicolas Colombain <nicolas.colombain@armadeus.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>