My idea of plugging a RC2014 bridge directly onto a Sega Master System
cartridge doesn't work. The SMS eats all I/O addr space, we can't use
it. Therefore, this naive idea, in the emulator, of reusing sdc.c in
sms.c as-is, doesn't work either.
I'll have to find another way of communicating to a SPI device on the
SMS. I'll probably do it through a controller port. Meanwhile, I need
to decouple SPI from SDC in the emulator code so that I can reuse
sdc.c. This is what is done here.
