[parseopt] Allow "prompt" command to accept character literals for --key

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/image/script.c

@@ -302,7 +302,7 @@ struct prompt_options {
 /** "prompt" option list */
 static struct option_descriptor prompt_opts[] = {
 	OPTION_DESC ( "key", 'k', required_argument,
-		      struct prompt_options, key, parse_integer ),
+		      struct prompt_options, key, parse_key ),
 	OPTION_DESC ( "timeout", 't', required_argument,
 		      struct prompt_options, timeout, parse_integer ),
 };