[new uImage] Add autostart flag to bootm_headers structure

The autostart env variable was dropped as part of the initial new uImage
cleanup.  Add it back here so the arch specific code can decide if it
wants to really boot or not.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Marian Balakowicz <m8@semihalf.com>