In order to reuse the support for the u-boot,spl-boot-order property
from the rk3399, we split it into a reusable module that can be
included by the SPL code for any of our boards.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>