better support for PKI authentication for ssh

Instead of just leaving it commented out, let's use ~/.config/.ssh/authorized_keys for handling ssh key authentication.

This should allow users to just add their public keys to the device, then be able to connect without password authentication further.
Also added a note to indicate that if users wish to restrict authentication to key-only (as would be wise in this case, as root's password is locked to "openelec", how they can go about changing it.