Currently we need to build one U-boot image for each of the wandboard
variants: quad, dual-lite and solo.

By switching to SPL we can support all these variants with a single binary,
which is very convenient.

Based on the work from Richard Hu.

Tested kernel booting on the three boards.

Signed-off-by: Richard Hu
Signed-off-by: Fabio Estevam
Tested-by: Vagrant Cascadian
Reviewed-by: Stefano Babic