buildroot

fs/ubi: use namespace for internal variable

UBINIZE_CONFIG_FILE_PATH is only used inside this file, so rename it to
start with UBI_, following the namespace convention already used by
common packages.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>