allow use of old radio.img name too, to build incrementals correctly

Change-Id: I1c630e8db8ca9baa141d10fa22c664a992f5a0d5
diff --git a/releasetools.py b/releasetools.py
index e13ad8e..cf561b7 100644
--- a/releasetools.py
+++ b/releasetools.py
@@ -20,7 +20,7 @@
 def FindRadio(zipfile):
   matches = []
   for name in zipfile.namelist():
-    if re.match(r"^RADIO/radio[.].+[.]img$", name):
+    if re.match(r"^RADIO/radio[.](.+[.])?img$", name):
       matches.append(name)
   if len(matches) > 1:
     raise ValueError("multiple radio images in target-files zip!")