Small fixes to the packaging script

* Add a .tag file to the archive that contains the sha1 the package was created from. This is useful for diagnostics.
* Use bash and enforce error handling (it's easy to accidentally include bashisms)

Change-Id: I8141b10d50bbf55a470d5a84ed0ae801742b57dc
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>