Building distro packages is great, however if they aren't built in the
correct environment with the associated dependencies, then they won't
work properly on those distros.
This patch adds an `mkosi` based image building environment that builds
the packages in their respective distros, and then copies them out into
our releases directory.
You'll now want to `make tag && make mkosi && make release` to get a new
release out. We use a small hack to trick the `make release` portion to
not re-build the distro packages if they're already present in the
releases/ directory for that version.
This commit depends on a very recent version of mkosi (it was tested
with git master) and also depends on two currently unmerged patches:
https://github.com/systemd/mkosi/pull/363 and
https://github.com/systemd/mkosi/pull/365
a4db9fc8e5