Add a new function which compares the stamp filename we want (including
taskhash) with what is in the stamp directory (using the clean mask).
This tells us which stamp files are stale and are due to be rerun.
(Bitbake rev: b126a53882d202e4df0f9661303355c9fe9ec80e)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>