ui/ncurses: Re-add autoboot selection to config screen

The new boot order interface removed the autoboot enable/disable buttons.
However this means the only way to disable autoboot is to remove all
options from the boot order list, or know that you can manually set the
`auto-boot?` flag in nvram.

This re-introduces an autoboot enable/disable widget so that autoboot
can be disabled without losing an existing boot order. The translated
strings for "Autoboot" are salvaged from earlier use, and two new
untranslated strings are introduced.

Signed-off-by: Sam Mendoza-Jonas <sam@mendozajonas.com>