cmd: Expose a Kconfig option to enable UBIFS commands

Create a new Kconfig entry to allow CMD_UBIFS selection from Kconfig and
add an hidden LZO option that can be selected by CMD_UBIFS.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Jagan Teki <jagan@openedev.com>