classes/sstate: add a mode to error if sstate package unavailable
If BB_SETSCENE_ENFORCE is set to "1" and an sstate package fails to
download outside of the whitelist specified by
BB_SETSCENE_ENFORCE_WHITELIST, then fail immediately so you can tell
that the problem was caused by failing to restore the task from sstate.
Part of the implementation of [YOCTO #9367].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
