boot/uboot: Add support for dtb.img format

Allow to generate u-boot-dtb.img. This is the same format
as u-boot.img, however, it contains a built-in device-tree.
This file is being loaded by the U-Boot SPL.

Signed-off-by: Jan Viktorin <viktorin@rehivetech.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>