image.bbclass: Separate out image generation into a new task, do_image
I've heard complaints from people trying to create more interesting image
types about how hard it is to understand the rootfs/image generation code
and that its a pain to develop/test/debug.
Having looked at it myself, the internal construction of shell functions which
then gets passed into a multiprocessing pool is rather convoluted and it places
rather odd constraints on when variables are expanded. Its therefore no wonder
people find it confusing/complex.
This patch starts the process of splitting this up by separating out image
generation from the do_rootfs task into a new do_image task.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
