Use the generic spi send/receive code on Linux. Update the replicape
code to use the updated command format.
Also, update the replicape code to turn off the stepper motors on a
shutdown event.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Add support for sending SPI messages to devices via the standard Linux
SPI interface. This can be used to configure the shift registers on
Replicape boards.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>