This class configures an onion service to access the SSH server over
Tor. This is useful when you have a box behind NAT or some firewall
that is broken or unknown, and you need to get a rescue shell on the
host.

With this, you give an operator a thumb drive, who only needs to
figure out how to boot into GRML, and then after a while you get a
shell, pretty much regardless of where the box is.

This is not enabled by default, naturally, otherwise the secret key
would leak in default GRML builds: this is solely designed to be run
in an ad-hoc, one-time fashion. It also generates the SSH keys for the
same reason: those are shown in the build logs and can be used to
authenticate the remote host (a redundant measure to the onion service
name, of course).

I also enable `DEFAULT_BOOT_OPTIONS=ssh` in my builds, but that hasn't
been done here (although maybe it's possible to enable that in the
class? to be investigated).

Finally, another shim is required here to inject a valid SSH public
key in the image, so you can login over SSH. In my case, I have an
extra CLASS that only has this one script which does:

    gpg --export-ssh-key [email protected] | tee -a $target/root/.ssh/authorized_keys

This could also be folded in the TOR class, but I'm not sure how to do
variables yet, so that's not yet standardized.