SD Card driver

Load range: 602-616

This driver is designed for a ad-hoc adapter card that acts as a
SPI relay between the z80 bus and the SD card. That adapter is  
expected to pull CS low when something is written to SDC_CSLOW,
high on SDC_CSHIGH and to initiate a SPI exchange when a byte is
written to SDC_SPI, the result of that exchange being fetched   
with a read to SDC_SPI.

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