Integrate meta-guf-distro/on_uboot_to_include

Commit: https://gitlab.com/garz-fricke/yocto/layers/meta-guf-distro/-/commit/5625f2b889675d1ab5e7e2ca31dfd72ecf28f555

distro:on_uboot: Move functio to include file

When 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.