bitbake: runqueue: Improve setscene task handling logic

bitbake: runqueue: Improve setscene task handling logic

The previous tasks_covered and tasks_notcovered were basically unstable
data structures. We couldn't always tell whether tasks should be covered
or not when trying to repair the sturcture if sstate tasks reran.

In the end its simpler to throw the lists away and rebuild them based upon
current data rather than trying to patch it adhoc. This turns out to be
simpler and much more reliable and I've much more confidence in this code.

(Bitbake rev: 52ee2ba2c617d928569f5afa404925c8b6f317bc)

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