[parseopt] Add parse_timeout()

Parsing a timeout value (specified in milliseconds) into an internal
timeout value measured in timer ticks is a common operation.  Provide
a parse_timeout() value to carry out this conversion automatically.

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

src/image/script.c

@@ -361,7 +361,7 @@ struct prompt_options {
 	/** Key to wait for */
 	unsigned int key;
 	/** Timeout */
-	unsigned int timeout;
+	unsigned long timeout;
 };
 
 /** "prompt" option list */
@@ -369,7 +369,7 @@ static struct option_descriptor prompt_opts[] = {
 	OPTION_DESC ( "key", 'k', required_argument,
 		      struct prompt_options, key, parse_key ),
 	OPTION_DESC ( "timeout", 't', required_argument,
-		      struct prompt_options, timeout, parse_integer ),
+		      struct prompt_options, timeout, parse_timeout ),
 };
 
 /** "prompt" command descriptor */