QA test individual Whitebox unit.

* Function to enable the built-in DDS.
* Add humble beginnings of a fixed-point DSP library.
* Track TX chain statistics and display in a debugfs file.
* Mount debugfs on bootup.
* Auto-tx module parameter auto enables the transmit chain
  when the fifo is almost full.
* Exciter / RF Sink can transfer individual samples without
  DMA when it makes sense.
* Fixed handling of interrupts in the write character
  driver method.
* Add TX FIFO tests to help ferrit out the error with my HDL.
* Fixed DUC tests to use new CIC filter.
* Add more driver transfer tests.
* Implement pdma_busy (albeit poorly).