mkimage: adding support for Davinci AIS image

Some Davinci processors supports the Application
Image Script (AIS) boot process. The patch adds the generation
of the AIS image inside the mkimage tool to make possible
to generate a bootable U-boot without external tools
(TI Davinci AIS Generator).

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Wolfgang Denk <wd@denx.de>