bitbake: siggen: Add new unitaskhashes data variable which is cached

We need to preserve unihash task hashes between runs. Use the new SimpleCache
class to create such a class within the signature generator which is loaded
at init time and saved when builds complete. The default is unpopulated but
metadata sig handlers can populate this cache.

(Bitbake rev: 1f326f2c29c2664a5daaeeb0c1fd332630efbdba)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/runqueue.py

@@ -1516,6 +1516,7 @@ class RunQueue:
             self.dm_event_handler_registered = False
 
         if build_done and self.rqexe:
+            bb.parse.siggen.save_unitaskhashes()
             self.teardown_workers()
             if self.rqexe:
                 if self.rqexe.stats.failed: