prefer releasetools.py from target_files zip

If the target_files zip for the target build contains a
META/releasetools.py (which it has since Nov 2013), prefer that over
using a releasetools.py from the local client.

Explicitly specifying the device-specific extensions path via
command-line options takes priority over both of the above mechanisms.

Change-Id: Ia068b0e2e06ede7da89ebe4315cdec592eb8995e

tools/releasetools/common.py

@@ -756,6 +756,7 @@ class DeviceSpecificParams(object):
           if x == ".py":
             f = b
           info = imp.find_module(f, [d])
+        print "loaded device-specific extensions from", path
         self.module = imp.load_module("device_specific", *info)
       except ImportError:
         print "unable to load device-specific module; assuming none"