ifdtool: Const-correct and redesign some functions

Const-correct some functions which do not write back,
and use pointers to access existing region_t variables.

The last changeset is dismantled this time.

This commit is only focused on const-correctness.

Change-Id: I6d05418c8b32fa31dcd038a3e56f9aefe13fa9c4
Signed-off-by: Bill XIE <persmule@gmail.com>
Reviewed-on: https://review.coreboot.org/21288
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/ifdtool/ifdtool.h

@@ -140,6 +140,6 @@ typedef struct {
 } region_t;
 
 struct region_name {
-	char *pretty;
-	char *terse;
+	const char *pretty;
+	const char *terse;
 };