Integrate meta-seconorth-distro/add-manifest-and-testdata-files-to-release-artifacts

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Commit: seco-ne/yocto/layers/meta-seconorth-distro@a316e95e

Refactoring: remove DISTRO_IMAGES variable

DISTRO_IMAGES was passed to the CI package stage only to add the
corresponding manifest and testdata.json files as well. Add these
directly to DISTRO_RELEASE_ARTEFACTS and remove DISTRO_IMAGES variable.

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Commit: seco-ne/yocto/layers/meta-seconorth-distro@ba99e05b

Refactoring: remove DISTRO_INSTALL_SCRIPTS

This variable is not needed anywhere else, so we can use its value
directly.

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Commit: seco-ne/yocto/layers/meta-seconorth-distro@1846382a

Refactoring: append to DISTRO_IMAGES for FNGSystem instead of redefining it

${IMAGE_LINK_NAME}.cpio.gz is part of DISTRO_IMAGES automatically due to
its original definition in seconorth-distro.inc. Append
${IMAGE_LINK_NAME}.tgz to make clear that this is an addition compared
to the Yocto build.