do_autoboot_countdown changes autoboot state if the user presses m for
menu or ctrl+c to abort during count down. This is an internal detail
and doesn't need to be reflected in the state of the global variable.

This will improve UX when exporting the variable in the follow-up
commit, because on a regular boot it $autoboot will expand to countdown
instead of abort/boot/menu dependent on prior user input.

Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>