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