x86: samus: Add a target to boot through TPL

Add a version of samus which supports booting from TPL to SPL and then
to U-Boot. This allows TPL to select from an A or B SPL to support
verified boot with field upgrade.

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