Autotest: fix githash on python 3 for firmware html page

Tools/autotest/autotest.py

@@ -484,6 +484,8 @@ class TestResults(object):
         self.githash = util.run_cmd('git rev-parse HEAD',
                                     output=True,
                                     directory=util.reltopdir('.')).strip()
+        if sys.version_info.major >= 3:
+            self.githash = self.githash.decode('utf-8')
         self.tests = []
         self.files = []
         self.images = []