Remove Dunfell Support and Layer Compat, Kernel 5.10 and introduce KED_DISTRO_CODENAME

Keeping the compatibility with Dunfell requires quite a bit of an overhead and makes future changes more difficult. There is not much gain in continuing to support Dunfell and we want to encourage people to update their BSPs and not stay with the old release. Therefore let's remove this altogether.

This is a somewhat breaking change and should be merged for v6.2 but not included in a patch release 6.1.2 (see https://git.kontron-electronics.de/sw/ked/plan/plan-yocto/-/issues/1312). Anyway there are no known users of meta-ked-bsp + Dunfell.

CC: @estoll