SD Card driver

Load range: 603-616

This driver is designed for a ad-hoc adapter card that acts as a
SPI relay between the z80 bus and the SD card. It requires a SPI
driver providing (spix), (spie) and (spid), which oh surprise!
is included in Collapse OS at B595.

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