tools: mkimage: Extend mkimage to also include pmufw

tools: mkimage: Extend mkimage to also include pmufw

The patch is adding external pmufw "Platform Management Unit firmware"
to boot.bin image. Boot.bin is a Xilinx format which bootrom is capable
to read and boot the system. pmufw is copied to the header data section
follows by u-boot-spl.bin. pmufw is consumed by PMU unit (Microblaze)
and SPL runs on a53-0.

This is generated command line when PMUFW_INIT_FILE is setup.

./tools/mkimage -T zynqmpimage -R ./"" -n
./"board/xilinx/zynqmp/pmufw.bin" -d spl/u-boot-spl.bin spl/boot.bin

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>