raspberrypi-sys-mods.sshswitch.service

[Unit]
Description=Turn on SSH if /boot/ssh is present
ConditionPathExistsGlob=/boot/ssh{,.txt}
After=regenerate_ssh_host_keys.service

[Service]
Type=oneshot
ExecStart=/bin/sh -c "systemctl enable --now ssh && rm -f /boot/ssh ; rm -f /boot/ssh.txt"

[Install]
WantedBy=multi-user.target