Integrate gitlab-ci/report-image-diff-rework and 1 more

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@bd4caeae

report_image_diff: rework variables extraction

The current solution makes too much assumptions about the pipeline and needs
to reimplement some of the gitlab yaml interpretion.

The new version of the script extracts MACHINE and DISTRO from the jobs themselves:
- MACHINE is part of the build job name ("build-$MACHINE");
- DISTRO is printed by bitbake and can be found in build log.

Typos

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@cbf168eb

build:credentials: Fix error when storing credentials in cache

The credentials helper wasn't storing the credentials in the cache.
Seemingly, the echo added whitespaces at the beginning of the lines
which can't be interpreted by the git credential approve command.
Also the USERNAME was renamed to TOKEN_USER because the variable is
already filled with the system users name.