Makefile: Add target for combined spl/u-boot.bin & u-boot.img

This new make target "u-boot-img.bin" consists of the U-Boot
SPL image with the real, full-blown U-Boot image directly
attached to it. The full-blown U-Boot image has the mkimage
header included, with its load-address and entry-point.

This will be used by the upcoming a3m071 MPC5200 board port.

Signed-off-by: Stefan Roese <sr@denx.de>