SPL: Enable use of custom defined U-Boot entry point

By setting CONFIG_SYS_UBOOT_START boards can now use a different entry
point for their U-Boot image. So the U-Boot entry point is not fixed
to CONFIG_SYS_TEXT_BASE any more.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Tom Rini <trini@ti.com>