extract_utils: extract(): rename DEST variable to VENDOR_REPO_FILE

* This denotes the path of the file that results from the extraction
  process, relative to the "proprietary" directory.
* This is a cleanup patch.

Change-Id: I38e759bb6ed697f0a31ca35a7aa9b9b92f8b6793
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>