Integrate meta-guf-distro/fix-fngsystem-release-artefacts-list

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Commit: https://gitlab.com/garz-fricke/yocto/layers/meta-guf-distro/-/commit/81d6c95dbcffa48bb61c91eff0a7dde63bd0863b

imx8mguf:release artefacts: Fix append syntax so all needed artefacts are copied

Use _append_mx8 instead of +=, as the previous way didn't really
appended but overwrote the variable content. This leaded to some missing
files in the release.