Tools: fix appending to bytearray in uploader.py for Python 3

Tools/scripts/uploader.py

@@ -175,11 +175,11 @@ class firmware(object):
             self.extf_image = None
         # pad image to 4-byte length
         while ((len(self.image) % 4) != 0):
-            self.image.append('\xff')
+            self.image += bytes(0xFF)
         # pad image to 4-byte length
         if self.extf_image is not None:
             while ((len(self.extf_image) % 4) != 0):
-                self.extf_image.append('\xff')
+                self.extf_image += bytes(0xFF)
 
     def property(self, propname, default=None):
         if propname in self.desc: