archive-release: Update EXTERNAL_TOOLCHAIN path.

* Previously MEL were installed in mel folder under CodeBench. In
* MEL2013.11 release MEL is installed in mel/version_number folder.
* So toolchain is present at 2 level up where MEL actually present.
* So updated the EXTERNAL_TOOLCHAIN variable.

Signed-off-by: Noor Ahsan <noor_ahsan@mentor.com>