Include layers-integration.yml file

* The file is intended to contain all the automation tasks to be
executed by the projects used in the Yocto layers
* The steps are the main steps that this CI file contains:
  - Every time a MR is opened on a project an integration branch is opened
 on the layer which updates the SHA of the commit on the
'conf/SRCREV.conf' file referring to correct bb recipe
  - A MR is automatically opened on the layer master branch, that triggers all the
downstream job of the manifest-integration file, which is left untouched
  - A check job checks that the layer project branch is up-to-date with
the commit revision
  - Once the MR is accepted on the project, the one in the layer is
merged and closed by default after having tried a rebase.
* Minor bug fix on gitlab-ci.yml file