x86: Allow building an SPL image for coreboot

Coreboot runs in 32-bit mode and cannot run a 64-bit U-Boot. To get around
this we can build a combined image with 32-bit SPL and 64-bit U-Boot. Add
a build rule and binman definition for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>