Comment 3 for bug 1355286

Out of curiousity, why does phablet-shell not add the key it needs to
authorized_keys instead of completely overwriting it.
This is what the SDK does:

echo $KEY >> /home/$USERNAME/.ssh/authorized_keys2

Also we do not use keys per device anymore but create one key in ~/.config/ubuntu-sdk/ubuntudevice_id_rsa
which is used for all devices.