ar71xx: factory image generation for WNDR4300

Add factory image generation using UBI/UBIFS. The rootfile is
mounted rw, no overlay support yet.

Just in front of the rootfs (still on the kernel partition),
a fake header needs to be in place to make stock U-Boot happy.
This fake header tells the system rootfs is 4 bytes long. The
first for bytes of the rootfs is an UBI magic (ASCII UBI#).
ar71xx: Empty whole rootfs partition on WNDR4300

UBIFS needs the whole partition to be empty upon first mounting.
Since the bootloader only flashes the used amount of the UBI
image, this patch padds the image with 0xff.