bootimg/image-vm/image-live: Improve image dependencies

bootimg/image-vm/image-live: Improve image dependencies

Now that image generation consists of separate tasks, we can improve
the dependencies in these classes. We can also incorporate the tasks
within do_image_complete instead of do_build so we have a specific task
for when the deploy directory is populated with images.

This fixes various expectations about do_image_completed and the way
I'd tried to use it to fix some sanity test problems.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>