mkimage: Fix argument parsing with signature comment

Inform getopt that '-c' requires a parameter.

Fixes: a02221f2 ("mkimage: Convert to use getopt()")
Signed-off-by: Karl Beldan <kbeldan@baylibre.com>
Reviewed-by: Tom Rini <trini@konsulko.com>

tools/mkimage.c

@@ -140,7 +140,7 @@ static void process_args(int argc, char **argv)
 	int opt;
 
 	while ((opt = getopt(argc, argv,
-			     "a:A:b:cC:d:D:e:Ef:Fk:K:ln:p:O:rR:qsT:vVx")) != -1) {
+			     "a:A:b:c:C:d:D:e:Ef:Fk:K:ln:p:O:rR:qsT:vVx")) != -1) {
 		switch (opt) {
 		case 'a':
 			params.addr = strtoull(optarg, &ptr, 16);