BaseTools: Improve the method of checking queue empty

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2807

The Queue.empty() method is not reliable in the multiple
process runtime environment. This patch uses a new method
to check if all modules are processed and workers need
to be stopped. That is to add a None item at the bottom
of the queue. Worker check if it gets that None item to
know if all the module is processed.

Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Cc: Lucy Yan <lucyyan@google.com>

Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/Python/build/build.py

@@ -1217,7 +1217,7 @@ class Build():
             mqueue = mp.Queue()
             for m in AutoGenObject.GetAllModuleInfo:
                 mqueue.put(m)
-
+            mqueue.put((None,None,None,None,None,None,None))
             AutoGenObject.DataPipe.DataContainer = {"CommandTarget": self.Target}
             AutoGenObject.DataPipe.DataContainer = {"Workspace_timestamp": AutoGenObject.Workspace._SrcTimeStamp}
             AutoGenObject.CreateLibModuelDirs()
@@ -2174,6 +2174,7 @@ class Build():
             data_pipe_file = os.path.join(Pa.BuildDir, "GlobalVar_%s_%s.bin" % (str(Pa.Guid),Pa.Arch))
             Pa.DataPipe.dump(data_pipe_file)
 
+            mqueue.put((None,None,None,None,None,None,None))
             autogen_rt, errorcode = self.StartAutoGen(mqueue, Pa.DataPipe, self.SkipAutoGen, PcdMaList, cqueue)
 
             if not autogen_rt: