Add u-boot-pad.bin target to the Makefile

Samsung's S3C24XX SoCs need this in order to generate a binary image
with a padded SPL concatenated with U-Boot.

Signed-off-by: José Miguel Gonçalves <jose.goncalves@inov.pt>
[scottwood@freescale.com: fixed prereq of u-boot.ubl]
Signed-off-by: Scott Wood <scottwood@freescale.com>
--
v2: Removed spl/ prefix from u-boot.ubl prerequisite.