uboot: add a new binary format for u-boot.img

For some platforms like OMAP, a new binary format is now being used
for u-boot: u-boot.img. It is basically u-boot.bin which has been
processed with mkimage.

Signed-off-by: Nicolas Dechesne <n-dechesne@ti.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>