#339 Provide --enable-xxx-port feature

[Brian Ruthven]

For building from source, I'm only interested in the Z80 port, so I have to specify many --disable-xx-port options to turn off all the others I don't want. Ideally I'd like to say --enable-z80-port which would imply all other --disable-xx-port automatically.

When a new port is added, I have to specify an additional --disable-xx option, else I waste time either trying to build something which fails (such as when the picXX ports were added, requiring something I didn't have, or more recently the r2k port), or simply takes time and disk space unnecessarily for a port I'm never going to use.