remember device-specific releasetools extensions in target-files

Store the location of the releasetools extensions in the target-files
zip, and make ota_from_target_files use that stored location by
default (though it can still be overridden with -s if desired).

tools/releasetools/common.py

@@ -440,7 +440,7 @@ class DeviceSpecificParams(object):
 
     if self.module is None:
       path = OPTIONS.device_specific
-      if path is None: return
+      if not path: return
       try:
         if os.path.isdir(path):
           info = imp.find_module("releasetools", [path])