Implement audio API

[thejpster]

For sending audio to the CODEC and setting the mixer

[thejpster]

#85 initialises the CODEC. Need a ringbuffer we can DMA from. Perhaps around 800 samples, which is one frame of video at 60 Hz. Will eat some TPA, but that's OK.

[thejpster]

Need to allow the OS to control the audio sample rate.