MANIFEST_PROJECT, MASTER_BRANCH_MANIFEST, MASTER_BRANCH_PROJECT

The variables CI_PARAM_IMAGE, CI_PARAM_DISTRO, CI_MACHINE have been
defined outside the job in the '.gitlab-ci.yml' file. This has a lower
precedence then variables defined inside the job, but it seems that
these variables are used in the scope of 'Trigger variables' when
available in the trigger job. These override the value set in the
template. As main results this made FNGSystem jobs build the normal
yocto.

* Move all build, deploy and test stubs to the manifest-build.yml
* Create a new job to generate and trigger the build jobs dynamically
* Add the base jinja2 file for the build jobs.
* Add initial docs for the manifest pipeline
* Remove unused files

BCS 746-000636

Rename variable JENKINSGUF_SSH_PRIVATE_KEY / SSH_PRIVATE_KEY to
GITLAB_PRIVATE_KEY on this occasion, because it contains a private key
that was generated exclusively for this use case. The according public
key has been added as a deploy key in GitLab to all repositories that
this repository needs access to.

Add more detailed documentation concerning this configurationdirectly in
the gitlab-ci files.

I did a test setup in the azure cloud. These are the notes how this was
done.

BCS 746-000594

* Add a draw.io diagram that illustrates the flow of the CI pipelines
across the manifest and layer repos
* Add a png export of the diagram

* Add a draw.io diagram that illustrates the flow of the CI pipelines
across the manifest and layer repos