[Coco] Stand-alone DriveWire devices.

[James Dessart]

John's suggestion of MMC just reminded me that MMC cards can also operate
in SPI mode, making it very easy to interface them to uCs. I even have
some code at home that I found somewhere that does it.

With the cost of MMCs, it should be fairly straightforward and cheap to
make a serial-based disk device for the CoCo, or really any machine that
works as a DriveWire client.

What we need now are system-level RBF drivers for Mac OS X and Windows. :)
the os9tools are great, but it'd also be nice to manipulate the volumes
directly.

James