Add CI bot to automatically update Yocto meta layers

This contains:

• Preparation for the bot implementation
• A job template that implements a simple bot to
  • Bump the layer revisions
  • Push the changes to a feature branch
  • Create a merge request

This was tested by adding the following code in the build directories .gitlab-ci.yml:

meta-bump-bot:
  extends: .meta-bump-bot

To give the pipeline push access to the repo and API permissions to create a MR you need to add a project access token to your build repo and expose it via a masked CI variable with the key PROJECT_ACCESS_TOKEN.

Cc: @jbauknecht