classes/image: move image_qa to between rootfs and image

classes/image: move image_qa to between rootfs and image

It was noticed that do_image_qa is useless when rm_work is enabled as the rootfs
directory is deleted before image_qa is called.

This indicates that image_qa is incorrectly scheduled as it failing should mean
images don't get generated, so move it between do_rootfs and do_image.

Also, add a little bit more documentation to the comments.

Signed-off-by: Ross Burton <ross.burton@intel.com>