build system: order per-region files to optimize placement success

build system: order per-region files to optimize placement success

Make sure that files with a fixed position are placed first (whose order
doesn't matter: either they collide or they don't), then all aligned
files (where we just hope that the right thing happens) and finally the
files with no further requirements (again, hope).
It's still a pretty good heuristic given a typical coreboot image.

The global sorting that happens earlier in the build flow will be
removed in the future to make room for per-region requirements.

Change-Id: I269c00b2ece262c95d310b76a6651c9574badb58
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/16137
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>