trs80: serial debug

This makes the serial debug work in the emulator. It's not sufficient for
doing serial debug on a real TRS80
trs80, hd: Make the hard disk driver work and add swap

With this we can now swap on the TRS80 platform and things seem to work
acceptably. Disk size and other properties are hardcoded in ways that will
need fixing. Partitioning support also needs to be added.