Fix failing systemd ssh.service

Fix failing systemd ssh.service

SSH needs host keys which previously were generated by
ssh-keygen.service. But the generation of the host keys is not a
"service" and must only be done when no host keys exist.

Also ssh-bootoption.service was aliased with sshd.service which itself
depends on the kernel option "ssh" which prevents starting of ssh. So I
removed the alias entry.