[Buildroot] boot/uboot: allow multiple target selection

[Petr Kulhavy]

Hi,

I'm sending two improvements to boot/uboot.

The first patch is a minor fix to make sure the SPL target is always build by
the make process.

The other patch changes the current "choice" type of selection of the U-boot
target format to a multi-selection. This might be useful for generating
multiple uboot image formats at once. E.g. one for serial load (which typically
contains some RBL formatting) and one to be stored on a filesystem or in the
flash. So far this was not possible to do.

Any comments are welcome.

Thanks
Petr