[parseopt] Allow "0x"-prefixed hexadecimal values in integer-valued options

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

src/core/parseopt.c

 	assert ( text != NULL );
 
 	/* Parse integer */
-	*value = strtoul ( text, &endp, 10 );
+	*value = strtoul ( text, &endp, 0 );
 	if ( *endp ) {
 		printf ( "\"%s\": invalid integer value\n", text );
 		return -EINVAL;