Fix env.sh

* Fix the check for the .gitmodules to be done on the right path. Now the
  "source env.sh" should work flawless when called from this repository
  directly or this repository as a submodule.
* Fix a typo on unset zsh got. bash was silent about it.

Change-Id: I534fd8e3d85736620708421c23d5cea3ded40b28