Integrate meta-guf-distro/on_uboot_as_class

Commit: https://gitlab.com/garz-fricke/yocto/layers/meta-guf-distro/-/commit/032c3c35444614758df271c17edc68a504488cd8

distro:on_uboot: Replaced with classed based implementation on_bootloader_uboot

if meta-guf-distro is in the layer list, but none of the guf distros was selected, the function was unknown in the packagegroups. Using an additional file cleans this up.

Also fixes a problem when DISTRO_BOOT_FILES was unset, which may happen in the same situation.