All you need to do is installing dropbear (a tiny SSH server) and BusyBox (a tiny shell, usually already installed) using apt-get. The initrd will be recreated automatically. Then reboot your computer.
ssh-keygen -o -a 100 -t ed25519
sshfp generates DNS SSHFP records from SSH public keys. sshfp can take public keys from a known_hosts file or from scanning the host's sshd daemon.