Integrate gitlab-ci/fix-reuse-of-outdated-integration-branch and 2 more

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@bcb6076b

CI: Add check if integration branch is on top of target branch before reusing it

The integration branch was reused even if there where new commits on the
target branch, which could lead into reverting some changes.
This adds a check, if the existing integration branch is directly based
on the target branch and deletes it if not.

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@3b5e9be0

CI: Pull new changelog_generator image, add token to the call.

BCS 746-000785

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@968886c0

Update README.md: Add description on the gitlab-ci CI and how to deploy