fstab: Only overlay /etc/ssh

There's really no sense in creating a writable copy of the whole `/etc`
hierarchy at `/run/etc/rw`.  Instead, let's just mount overlays at the
paths we want to make writable (which for now is only `/etc/ssh`).

overlay/usr/libexec/init-storage

@@ -28,6 +28,7 @@ format_dev() {
     mount "${dev}" "${tmpdir}" || exit
     btrfs subvolume create "${tmpdir}"/etc || exit
     mkdir -p "${tmpdir}"/etc/.work "${tmpdir}"/etc/rw || exit
+    mkdir -p "${tmpdir}"/etc/rw/ssh
     btrfs subvolume create "${tmpdir}"/var || exit
     btrfs subvolume create "${tmpdir}"/var/log || exit
     umount "${dev}" || exit