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.
The include file solution leaded to a duplicated include warning as recipes and the distro uses it.