ubifs: Modify ubifs u-boot wrapper function prototypes for generic fs use

Modify the ubifs u-boot wrapper function prototypes for generic fs use,
and give them their own header file.

This is a preparation patch for adding ubifs support to the generic fs
code from fs/fs.c.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Heiko Schocher <hs@denx.de>