image: Deploy images to IMGDEPLOYDIR

image: Deploy images to IMGDEPLOYDIR

Changed deployment directory from DEPLOY_DIR_IMAGE to
IMGDEPLOYDIR to make sstate machinery to do final deployment and
generate manifest.

Renamed variable deploy_dir to deploy_dir_image in selftest code
to avoid confusion with DEPLOYDIR variable.

Updated the code of rootfs.py:Rootfs class to use IMGDEPLOYDIR variable
as it's now used as a new deployment destination.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>