ifdtool: merge region_filenames with region_name(s)

There is no reason to keep a separate region_filenames array,
so I merge it into region_name(s).

This commit is one separated from the original I6d05418c.

Change-Id: I38489c6d3b3c161e9b0281188e6cdd0b62e38335
Signed-off-by: Bill XIE <persmule@gmail.com>
Reviewed-on: https://review.coreboot.org/21509
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/ifdtool/ifdtool.h

@@ -142,4 +142,5 @@ typedef struct {
 struct region_name {
 	const char *pretty;
 	const char *terse;
+	const char *filename;
 };