uboot: Add support for U-Boot SPL

SPL is a first stage bootloader. On pandaboard it supercedes x-loader,
and should now be used.

This patch ensures that either SPL or xloader can be selected.

A config variable has been added for the name of the SPL binary generated
during u-boot build. For most platform it is u-boot-spl.bin but not always.
It is MLO on OMAP for example.

Signed-off-by: Nicolas Dechesne <n-dechesne@ti.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Frank Hunleth <fhunleth@troodon-software.com>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>