autostart: unify duplicated logic into the bootm code

Rather than having a bunch of random commands handle autostart behavior,
unify the logic in a single place.  This also fixes building of these

different commands when bootm is disabled.

Acked-by: Matthew McClintock <msm@freescale.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>