New initialization scripts

These initialization script will create a build environment which has only
the needed meta layers from upstream. Other repos are ignored and not even
downloaded.

Google's repo tool is used to fetch upstream repos and it can then be
used to manage the repos while developing. This has also support for
using local mirror repos.

The required repos are defined in the manifest.xml file, which can be
overwritten using a machine specific manifest_<device>.xml file.

Task-number: QTEE-760
Change-Id: I12ed9a6fddceb4de1217baaebf46277c6ef7224f
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@digia.com>