[new uImage] Add support for new uImage format to mkimage tool

Support for the new uImage format (FIT) is added to mkimage tool.
Commandline syntax is appropriately extended:

mkimage [-D dtc_options] -f fit-image.its fit-image

mkimage (together with dtc) takes fit-image.its and referenced therein
binaries (like vmlinux.bin.gz) as inputs, and produces fit-image file -- the
final image that can be transferred to the target (e.g., via tftp) and then
booted using the bootm command in U-Boot.

Signed-off-by: Bartlomiej Sieka <tur@semihalf.com>