Revert "[new uImage] Add autostart flag to bootm_headers structure"

This reverts commit f5614e79.

The commit was based on a misunderstanding of the (documented)
meaning of the 'autostart' environment variable. It might cause
boards to hang if 'autostart' was used, with the potential to brick
them. Go back to the documented behaviour.

Conflicts:

	common/cmd_bootm.c
	common/image.c
	include/image.h

Signed-off-by: Wolfgang Denk <wd@denx.de>