Instead of including distribution specific files in the subimages,
let's have one common mkosi.pkgenv/ directory that handles all the
matching which is then included in the subimages.
This gives us more control on exactly how we do the matching.
Now that mkosi can automatically pick up its main configuration from
a mkosi/ subdirectory if it exists and there is no configuration in the
top level directory, let's make use of it to reduce the amount of clutter
in the top level directory of the repository.
This will also make it easier to install the mkosi configuration files as
part of the testing packages later on.
