Add a script to check which sstate artifacts would be installed by
building a given target - by default this is done with a separate
TMPDIR to ensure we get the "from scratch" result. The script produces a
list of tasks that will be restored from the sstate cache. This can also
be combined with BB_SETSCENE_ENFORCE* to check if sstate artifacts are
available.

The implementation is a little crude - we're running bitbake -n and
looking at the output. In future when we have the ability to execute
tasks from tinfoil-based scripts we can look at rewriting that part of
it to use that instead.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>