omap3: overo: Allow boot with kernel in UBI rootfs

If regular NAND booting fails to find a valid uImage in the
kernel partition in NAND, try to boot using a zImage and dtb found
in a UBI volume in the rootfs partition.  This is the NAND analog
of mmc zImage booting for device-tree based kernels.

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Signed-off-by: Arun Bharadwaj <arun@gumstix.com>