cbfstool: signed vs. unsigned fixes

Use the right data types to fix compiler warnings.

Change-Id: Id23739421ba9e4a35599355fac9a17300ae4bda9
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Reviewed-on: http://review.coreboot.org/1236
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins)

util/cbfstool/cbfstool.c

@@ -348,7 +348,7 @@ static void which_endian(void)
 
 int main(int argc, char **argv)
 {
-	int i;
+	size_t i;
 
 	if (argc < 3) {
 		usage();