ci: Add META_OVERRIDES_APPEND to extend META_OVERRIDES via (global) variable
Sometimes a feature branch in a meta layer depends on changes in other layers. The easiest way to create a matching pipeline is, to manually trigger a pipeline via the web UI and specify the dependencies in the META_OVERRIDES variable.
As META_OVERRIDES is set in the job, we can't extend it by overriding via global variable. Also we would need to duplicate the trigger layers override.
To circumvent these issues this extends META_OVERRIDES by META_OVERRIDES_APPEND. Setting the latter in a global variable allows to specify additional layer dependencies without the need to change .gitlab-ci.yml.