Communicating through SPI

Many very useful device are able to communicate through the SPI
protocol, for example, SD cards and AVR MCUs. In many cases,
however, CPUs can't "speak SPI" because of their inability to
bit-bang.

In most cases, we need an extra peripheral, which we can build
ourselves, to interface with devices that "speak SPI". We call
this peripheral a SPI relay.

The design of those relays depend on the CPU architecture. See
spi.txt in arch-specific folders for more information.

SPI Relay protocol

This protocol enables communication with a SPI relay. This
protocol is designed to support devices with multiple endpoints.
To that end, (spie) takes a device ID argument, with a meaning
that is up to the device itself. To disable all devices, supply
0 to (spie).

We expect relay devices to support only one enabled device at
once. Enabling a specific device is expected to disable the
previously enabled one.

(spie)    n --     Enable SPI device
(spix)    n -- n   Perform SPI exchange (push a number, get a
                   number back)

There is no SPI subsystem, but other subsystems depend on the
SPI protocol being fulfilled:

* SD Card subsystem (doc/sdcard)