fs/ubifs: factorize all the "depends on" into "if...endif" blocks

Factorize all the "depends on" into "if...endif" blocks

All the UBIFS options use "depends on BR2_TARGET_ROOTFS_UBIFS" but
we can simplify the config file by enclosing them in an "if..endif"
block.

Signed-off-by: Mark Jackson <mpfj@newflow.co.uk>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>