Add sshd-keygen service
Runs only on first boot (empty /etc/) and if /etc/ exists, is writable and is not a symlink. Allows a usr-only image with an empty /etc/ (firstboot or factory reset) to boot and have a working sshd out of the box. Given the firstboot condition is checked, it will be a no-op and gracefully skipped on normal systems where /etc/ exists and is created by the installer.