mkosi: move distro files to mkosi.default.d/

With this change, "mkosi build" will automatically build systemd for the
current distro without any further configuration. If people want to do a
cross-distro build by default, they can still create mkosi.default, but I
assume that this is relatively rare.

If people have symlinked mkosi.default to one of the files in .mkosi/, they'll
need to adjust the symlink.

(Building without configuration would always fail, since systemd has many many
required dependencies. I think it's nicer to do the most commonly expected
thing by default, i.e. rebuild for the current distro.)

Mkosi is nowadays packaged for most distros, so recommend installing of distro
packages as the primary installation mechanism.

docs/HACKING.md

@@ -34,13 +34,10 @@ possible, however. In order to simplify testing for cases like this we provide
 a set of `mkosi` build files directly in the source tree. `mkosi` is a tool for
 building clean OS images from an upstream distribution in combination with a
 fresh build of the project in the local working directory. To make use of this,
-please acquire `mkosi` from https://github.com/systemd/mkosi first, unless your
-distribution has packaged it already and you can get it from there. After the
-tool is installed, symlink the settings file for your distribution of choice
-from .mkosi/ to mkosi.default in the project root directory (note that the
-package manager for this distro needs to be installed on your host system).
-After doing that, it is sufficient to type `mkosi` in the systemd project
-directory to generate a disk image `image.raw` you can boot either in
+please install the `mkosi` package (if not packaged for your distro, it can be
+downloaded from https://github.com/systemd/mkosi). `mkosi` will build an image
+for the host distro by default. It is sufficient to type `mkosi` in the systemd
+project directory to generate a disk image `image.raw` you can boot either in
 `systemd-nspawn` or in an UEFI-capable VM:
 
 ```