siggen.py: make sure stamp directory existing

or else "bitbake -S" from scratch may report "No such file or directory" error.

Signed-off-by: Kevin Tian <kevin.tian@intel.com>

bitbake/lib/bb/siggen.py

@@ -137,6 +137,8 @@ class SignatureGeneratorBasic(SignatureGenerator):
         else:
             sigfile = stampbase + "." + task + ".sigbasedata" + "." + self.basehash[k]
 
+        bb.utils.mkdirhier(os.path.dirname(sigfile))
+
         data = {}
         data['basewhitelist'] = self.basewhitelist
         data['taskwhitelist'] = self.taskwhitelist