arm, davinci: Add support for generating AIS images to the Makefile

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Mike Frysinger <vapier@gentoo.org>