ivory-bsp-tests: DMAUART echoprompt hooked up properly
ivory-bsp-tests: DMAUART echoprompt hooked up properly

- take dmauart ostream, put it into a ringbuffer, periodically feed it
  byte by byte to the echoprompt
- prompt now follows backpressure protocol: keeps an internal buffer, emits
  only when previous buffer has finished sending.

GaloisInc/ivory-tower-stm32,GaloisInc/ivory-tower-stm32,GaloisInc/ivory-tower-stm32