cache: don't write out the cache unnecessarily

cache: don't write out the cache unnecessarily

If the only recipes's we reparsed this run were those flagged as not to be
cached, there's no point in re-saving the cache, as those items won't be
included anyway.

(Bitbake rev: 1e0c4dbcbec886a30b89f8b4bb365c3c927ef609)

Signed-off-by: Chris Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>