lualoader: Don't draw loader menu with autoboot_delay=-1

This was mostly a cosmetic issue. autoboot_delay=-1 is documented to bypass
the loader menu and immediately execute the boot command, but lualoader
would draw the menu and immediately execute the boot command. No interaction
was possible with the menu.

The fix lifts autoboot_delay processing out of menu.autoboot, which now
takes a delay and does nothing if no delay is specified. This lines up with
my expectations of menu.autoboot's usage from a third party, which may
want more control over the process than the default behavior.

PR: 231610
Approved by: re (gjb)