Makefile: Add possibility to set entry-point for u-boot.img

This patch enabled boards using the SPL framework to set
an entry point in the U-Boot mkimage image "u-boot.img".
Until now the entry point in the header has been set to 0.
By setting CONFIG_SYS_UBOOT_START in the board header, boards
can override this default location.

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

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