As the next step in splitting up do_image, move the pre and post processing
commands to separate tasks. This also creates the do_image_complete task
which acts as the end marker task for image generation.
(From OE-Core rev: 800528eaa421d451b596545125cb218e08989151)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>