add apkg-powered upstream packaging for deb, rpm

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:
https://gitlab.nic.cz/packaging/apkg/-/merge_requests/35

distro/README.md

@@ -0,0 +1,69 @@
+# BIRD upstream packaging sources
+
+This distro/ directory contains packaging sources initially copied from Debian
+and Fedora downstream repos.
+
+Files in this directory follow [apkg] conventions and apkg can be used to
+create BIRD packages for various distros directly from upstream sources as
+well as from upstream archives.
+
+[apkg]: https://apkg.rtfd.io
+
+
+## Create (source) package from current repo commit
+
+Following command should build source package for current distro directly
+from current repo state - run at top bird directory:
+
+    apkg srcpkg
+
+or build native packages directly:
+
+    apkg build
+
+or in case of disposable VM/container you can use faster direct host build
+
+    apkg build -Hi
+
+tools/make-dev-archive script is in charge of creating archive from source.
+
+
+## Create (source) package from upstream release
+
+Following commands can be used to clone upstream repo, download current upstream
+archive (tarball), and build Debian, Ubuntu, Fedora, CentOS, or OpenSUSE
+source package (depending on host system) using files in bird/distro:
+
+    git clone https://gitlab.nic.cz/labs/bird
+    cd bird
+    apkg get-archive
+    apkg srcpkg -a pkg/archives/upstream/bird-2.0.8.tar.gz
+
+To create native packages instead use `build`:
+
+    apkg build -a pkg/archives/upstream/bird-2.0.8.tar.gz
+
+Or to build packages directly in case of a disposable VM/container (faster, modifies system):
+
+    apkg build -Hi -a pkg/archives/upstream/bird-2.0.8.tar.gz
+
+
+## Build packages in openSUSE Build Service (OBS)
+
+tools/make-obs script can be used on Debian-based system to create OBS
+source package in pkg/obs directory ready to be uploaded:
+
+    cd bird
+    apkg get-archive
+    ./tools/make-obs
+    # result in pkg/obs
+
+You can also supply (upstream) archive to build from:
+
+    # or to use specified archive
+    ./tools/make-obs pkg/archives/upstream/bird-2.0.8.tar.gz
+
+
+## More Info
+
+Please see [apkg docs][apkg].