AVR chips often run at less than z80's system clock. This means that
our SPI relay needs to have its own clock to properly communicate
with it. This means that the delay between OUT and IN can't be
hardcoded to 2 NOPs anymore. It needs to be configurable.
Although the SPI Relay driver is RC2014-specific, the SD Card driver
is generic enough to be a subsystem. That's the second subsystem we
add and this warrants, I think, the formalization of a new concept:
protocols.