Suggested by Mathijs Kwik. ‘boot.systemd’ is a misnomer because systemd affects more than just booting. And it saves some typing.
Inspired by http://pkgs.fedoraproject.org/cgit/rng-tools.git/tree/rngd.service?id=27b1912b2d9659b6934fd4c887e46c13958e7e3c