openssh: Move sshdgenkeys.service to sshd.socket

sshd.socket launches the templated sshd@.service, so by moving the
sshdgenkeys.service to sshd.socket, key generation can start in advance
of a connection.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>