add build-farm support to Makefile

The `farm' target run `server', but after the server starts, also
builds clients (via `ssh') as specified in a configuration file.
A client can be a VirtualBox virtual machine, in which case
the client machine can be started and stopped automatically.

Most of the work is in `distro-build/drive-clients' (in the
"distro-build" package), and that's where the configuration-file
format and client-machine requirements are documented.