stand: Add "Loader needs updating" to the first menu item"

When the boot loader version is too old, add a warning to the boot menu
to maybe catch people's attention.

Use the correct loader code that adds an inactive highlighted menu item
indicating that an update is needed.

Sponsored by: Netflix
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D45890

(cherry picked from commit 0eac99f76ec31270f902cc2a0ff5ae4b5b606a65)
(cherry picked from commit 7cb65be96d47cbe0b740266bc633d272fc4c5e6b)