From rc(8):

  # Push the old seed into the kernel, create a future seed  and create a seed
  # file for the boot-loader.
  random_seed() {
        dd if=/var/db/host.random of=/dev/random bs=65536 count=1 status=none
        chmod 600 /var/db/host.random
        dd if=/dev/random of=/var/db/host.random bs=65536 count=1 status=none
        dd if=/dev/random of=/etc/random.seed bs=512 count=1 status=none
        chmod 600 /etc/random.seed
  }

* https://cvsweb.openbsd.org/src/etc/rc?rev=1.537

FreeBSD:

* https://svnweb.freebsd.org/base/head/libexec/save-entropy/

* https://www.freebsd.org/cgi/man.cgi?loader.conf(5) (see "entropy_cache_load")

OpenBSD's boot loader also injects entropy into the kernel.