Cleanup meson.build and don't install empty dir in /var on systemd systems

The main motivation for this is to help support the setup described in https://systemd.io/BUILDING_IMAGES/ and https://0pointer.net/blog/fitting-everything-together.html , and in particular, starting with a non existent /var which is created at boot time. The only commit which is actually relevant for this is 6d1a1cdf8126867f28dc772f4c782af4c1f9db5e , the others are just cleanup, which should not changing anything, if I haven't made a mistake.