build system: change addition order of files to go by region

Instead of adding each file in all requested regions, sort by region,
then by file.

This is in preparation of per-region file options
(eg. position, alignment)

Change-Id: Ide09a1c8840279380294a059bbd5d2f9f0cba780
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/16130
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>
1 file changed