testsuite: generalize mkosi support for other distros

testsuite: generalize mkosi support for other distros

Instead of using the mkosi.default symlink, use an env var passed from
the build system. We would need to pass the --default switch nonetheless
or change the symlink, making the git tree dirty.

Also, search for installed kernel headers in a way that's compatible
with more distros. On Fedora, for example, the
/usr/lib/modules/<kver>/build symlink is only available if there's a
kernel installed. We don't care about a kernel installed since we don't
need to boot it on a real machine: the only thing we need is the
kernel-devel package.