Makefile: Add u-boot.spr build target (SPEAr)

On x600 (SPEAr600) U-Boot is appended to U-Boot SPL. Both images are
created using mkimage (crc etc), so that the ROM bootloader can check
its integrity. Padding needs to be done to the SPL image (with
mkimage header) and not the binary. Otherwise the resulting image
which is loaded/copied by the ROM bootloader to SRAM doesn't fit.
The resulting image containing both U-Boot images is called u-boot.spr.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Amit Virdi <amit.virdi@st.com>
Cc: Vipin Kumar <vipin.kumar@st.com>