At present we only support a single loader of each time. Extra ones are
ignored. This means that only one BOOT_DEVICE_BOARD can be used in the SPL
image.
This is inconvenient since we sometimes want to provide several
board-specific drivers, albeit at different priorties. Add support for
this.
This should have no functional change for existing boards.
Signed-off-by: Simon Glass <sjg@chromium.org>