SD Card subsystem

Load range: B423-B436

This subsystem is designed for a ad-hoc adapter card that acts
as a SPI relay between the z80 bus and the SD card. It requires
a driver providing the SPI Relay protocol. You need to define
SDC_DEVID to specify which ID will be supplied to (spie).

Through that layer, this driver implements the SDC protocol
allowing it to provide BLK@ and BLK!.