Support for "headless", eg. Pico Audio

[tomgidden]

Although pico-examples a2dp_sink_demo works quite well unaltered, for completeness it'd be good to have a precompiled BT speaker firmware for the Pico Audio board, rather than having to do the whole SDK setup just to compile one image. I know this is all about the blinkenlights, but in lieu of a Pico Audio-specific downloadable to do the same thing, it seems straightforward.

I've done this brutally by adding a HEADLESS constant and #ifndef'ing the display-specific code. It could be done much cleaner, but I'd expect that'd require substantial refactoring to support an efficient "null" Display class.

I suspect there's some work being done capturing the audio buffer that might be unnecessary if it's not passed out to the Display/Effect code. Is btstack_audio_pico.cpp redundant without a display? If so, how to remove it?