lualoader: Don't execute menu.autoboot() for every opened menu

Attempt to autoboot when we open the default menu, and only when we open the
default menu. This alleviates the need for checking menu.already_autoboot,
because we're not trying to autoboot every time we open a submenu.

I note that escaping to loader prompt and going back to the menu (by running
require('menu').run() at the loader prompt) will happily work and not
re-initiate the autoboot sequence since "Escape to loader prompt" disables
the autoboot_delay.