build/siggen: Ensure a task signature file is generated for each task that is executed

Tracing down signature changes and dependencies is hard unless the complete task
chain can be evaultated. Since we have the data available at task exeuction
time, writing it to disk makes sense.

This change is suboptimal internal knowledge stamp_internal() has leaks into
make_stamp() and the concepts used by this code need revisiting but the change
brings enough usability benefit to warrant its inclusion despite that.

Addresses issue [YOCTO #1074]

(Bitbake rev: 139b8a625818225c358a1b8363518d7ed6913188)

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

bitbake/lib/bb/siggen.py

@@ -47,6 +47,9 @@ class SignatureGenerator(object):
     def stampfile(self, stampbase, file_name, taskname, extrainfo):
         return ("%s.%s.%s" % (stampbase, taskname, extrainfo)).rstrip('.')
 
+    def dump_sigtask(self, fn, task, stampbase, runtime):
+        return
+
 class SignatureGeneratorBasic(SignatureGenerator):
     """
     """