The comment above do_autoboot_countdown() states: "This function can be
called multiple times, it is executed only the first time.". Since
1973892533 ("startup: don't clobber original autoboot state") This is no
longer the case. Bring back the old and documented behaviour.

Fixes: 1973892533 ("startup: don't clobber original autoboot state")
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>