Add support for automotive SPI protocol

[ajn96]

The current buffer board firmware supports only the standard ADIS IMU SPI protocol. It would be useful to add "invisible" support for the automotive SPI protocol (see ADXC1500) w/ CRC (see ADXC1500). This will require a few tweaks:

1. Add code for generating SPI words based on the requested read/write, rather than simply passing transactions
2. Add logic for supporting writes. The automotive SPI protocol uses read16/write16, rather than the read16/write8 protocol used by the IMU product line. This can be worked around by adding logic to the IMU write reg routine which waits for two consecutive 8-bit writes to be requested, then performs a single 16-bit write.

Plan: Add two bits to IMU_SPI_CONFIG: AUTO_SPI and AUTO_SPI_WRITE16 to support this functionality