Integrate gitlab-ci/define-yamllint-explicitly

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@2b2bc220

Define yamllint job explicitly in each pipeline

The yamllint job was included in the pipeline implicitly via the
inclusion of common.yml. This is not very transparent. Make the common
definition a base job and extend it explicitly in each pipeline now.

Add infrastructure stage comment header into both yaml files.