Files in a single new distro/ dir allow apkg to build BIRD packages for
various distros directly from upstream sources as well as from upstream
archives.

Please see distro/README.md for more detail as well as apkg docs:

https://apkg.rtfd.io

I've used these files to build bird-2.0.8 on all currently supported
releases of following distros:

* Debian
* Ubuntu
* Fedora
* CentOS
* openSUSE

Please note that latest apkg with accumulated fixes for bird is needed:
packaging/apkg!35