Rework output to directly support 4A

When building for 4A, switch to new gstreamer-based ALSA output for
the RTL-SDR backend, and the Kingfisher backend now uses a gstreamer
pipeline for its loopback to either an ALSA or Pulse sink depending
on 4A or not.  Using gstreamer instead of direct ALSA output has the
benefit of transparently handling resampling to the M3ULCB hardware's
required 48 KHz sample rate for the RTL-SDR backend.

Change-Id: I2bfbf924927bb461cce88b04aba0e626f8d71215
Signed-off-by: Scott Murray <scottm@ghidorah.spiteful.org>

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