add signing checker script to releasetools

The check_target_files_signatures determines what key was used to sign
every .apk in a given target_files.  It can compare that signature to
that of another target_files (eg, the previous release for that
device) and flag any problems such as .apks signed with a different
key.
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 26f216d..0e17a5f 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -141,12 +141,15 @@
   BuildAndAddBootableImage(os.path.join(OPTIONS.input_tmp, "BOOT"),
                            "boot.img", output_zip)
 
-def UnzipTemp(filename):
+def UnzipTemp(filename, pattern=None):
   """Unzip the given archive into a temporary directory and return the name."""
 
   tmp = tempfile.mkdtemp(prefix="targetfiles-")
   OPTIONS.tempfiles.append(tmp)
-  p = Run(["unzip", "-o", "-q", filename, "-d", tmp], stdout=subprocess.PIPE)
+  cmd = ["unzip", "-o", "-q", filename, "-d", tmp]
+  if pattern is not None:
+    cmd.append(pattern)
+  p = Run(cmd, stdout=subprocess.PIPE)
   p.communicate()
   if p.returncode != 0:
     raise ExternalError("failed to unzip input target-files \"%s\"" %