grub2: add support for menu_show_once_timeout

Author: champtar

src/grub2/configs.d/14_menu_show_once.cfg

@@ -0,0 +1,8 @@
+# Force the menu to be shown once, with a timeout of ${menu_show_once_timeout}
+# if requested by ${menu_show_once_timeout} being set in the env.
+if [ "${menu_show_once_timeout}" ]; then
+  set timeout_style=menu
+  set timeout="${menu_show_once_timeout}"
+  unset menu_show_once_timeout
+  save_env menu_show_once_timeout
+fi