arm: kirkwood: Add CONFIG_SUPPORT_PASSING_ATAGS

Add CONFIG_SUPPORT_PASSING_ATAGS and friends to support legacy
image method of booting. Debian and OpenWrt installer use uImage
with appended DTB for these Kirkwood boards.

Signed-off-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>