For the steps in IMAGE_GENERATING, the patch consolidates them into
generate_image_async() to call.
(Bitbake rev: d4f2335e40d4d667847d3faed79bcee74baeea37)
Signed-off-by: Shane Wang <shane.wang@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>