Extend the DFU application to operate using SPI

[mbanth]

This issue follows on from xmos/sw_xvf3800#294 and #64 and covers:

• Extending the DFU application to allow operation using SPI
• Updating the corresponding design documentation, and
• Unit testing the extensions in the DFU application.

System testing of this extension occurs under xmos/sw_xvf3800#100.

[mbanth]

@LucianoMartin, please provide a Story Point value for this issue. Thanks.

[xluciano]

I assigned 3 story points, as we expect this issue to be mostly a copy of the code and documentation we would implement for I2C