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).
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index e758deb..27264dd 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -440,7 +440,7 @@
 
     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])