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

This new make target "u-boot-img-spl-at-end.bin" consists of the
the real, full-blown U-Boot image and the U-Boot SPL binary
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 lwmon5 PPC440EPx derivate board
port.

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