Ubuntu 14.04.3
Install dropbear SSH server with automated hooks for being included in an initramfs.
apt-get install dropbear
Remove default private/public key for login, clear dss key (would be regenererated otherwise), remove insecure 1024-bit RSA host key and generate a new 2048-bit RSA host key (we'll have to keep the DSA-key, dropbear won't start otherwise - make sure to use RSA when connecting):