firstboot: fix ssh service start issue at first booting

The SSH service fails at first boot due to missing host keys in /etc/ssh
even though new host keys are generated using 'ssh-keygen -A' since the
service keeps restart too quickly.

Therefore new approach is to disable the ssh service when building an
image, enable and start it after new host keys are generated in the
first boot script.

Signed-off-by: Dongjin Kim <tobetter@gmail.com>
Change-Id: Idee71e55017e72ec693ddf5e829c27da6343f031

fixups/S90-firstboot

@@ -26,3 +26,5 @@ __EOF
 
 systemctl enable firstboot.service
 systemctl disable ondemand
+
+systemctl disable ssh

overlay/root/firstboot.sh

@@ -24,6 +24,8 @@ resize2fs ${rootdev}
 rm -f /etc/ssh/ssh_host* && ssh-keygen -A
 
 sed -i "s/PasswordAuthentication.*/PasswordAuthentication yes/g" /etc/ssh/sshd_config
-service ssh restart
+
+systemctl enable ssh
+systemctl restart ssh
 
 exit 0