Use the new functionality in build.py to identify stale setscene tasks
and send an event to the metadata listing them. The metadata then
has the option of performing cleanup operations if it thinks that
appropriate.
(Bitbake rev: ef8c980a3ae92c168b7ca16a4d19cd38a9574761)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>