use bsdiff not imgdiff to compute radio diffs

Change-Id: Ib4f81eaf0e5229ea2a1c8662479780fafead7231
diff --git a/releasetools.py b/releasetools.py
index 056c7dc..b9dabdb 100644
--- a/releasetools.py
+++ b/releasetools.py
@@ -102,7 +102,7 @@
     if tf.sha1 == sf.sha1:
       print "radio image unchanged; skipping"
     else:
-      diff = common.Difference(tf, sf)
+      diff = common.Difference(tf, sf, diff_program="bsdiff")
       common.ComputeDifferences([diff])
       _, _, d = diff.GetPatch()
       if d is None or len(d) > tf.size * common.OPTIONS.patch_threshold: