SPI subsystem description

The XCSB SPI subsystem allows the use of both hardware and software driven SPI.

Software driven SPI is normally refered to at "bit banged". The hardware drivers are located int the library LIB/spi_lib.bas
The software (bit banged) drivers are located int the library LIB/spi_bb_lib.bas

These are very low level SPI drivers and are normally used by higher level functions such as SPI_25LC1024_read and SPI_25LC1024_write