Add target to show useful .ssh/config

[arichardson]

Something like

Host *cheribsd*
       User root
       AddressFamily inet
       HostName localhost
       IdentityFile ~/.ssh/insecure_id_ed25519
       ControlPath ~/.ssh/controlmasters/%r@%h:%p
       ControlMaster auto
       PasswordAuthentication no
       StrictHostKeyChecking no
       UserKnownHostsFile /dev/null

Host cheribsd-mips-purecap
        Port 12374
Host cheribsd-mips-hybrid
        Port 12375

etc.

Seems like there is an Include directive so maybe this should be created as ~/.config/cheribuild_ssh_config and users can choose to include it.