mobile: integrate GAPK based audio (voice) I/O support

This change introduces a new feature to the mobile application -
audio I/O support, which allows the user to speak right from the
host side running mobile through its ordinary mic and speakers.

The audio I/O is based on libosmo-gapk, which offers ALSA sound
sysyem support.  The libosmo-gapk's API implies to use the
processing chains and queue, which basically consist of a source
block, several processing blocks, and a sink block.  So, there
are two voice processing chains:

  - 'pq_audio_source' (voice capture -> frame encoding),
  - 'pq_audio_sink' (frame decoding -> voice playback).

Both of them exchange frames from two dedicated buffers:

  - 'tch_fb_ul' - a buffer for to be played DL TCH frames,
  - 'tch_fb_dl' - a buffer for encoded UL TCH frames.

In its turn, both buffers are served by a new gapk_io_dequeue()
function, which is being called within the mobile_work() loop.

Change-Id: Ib86b0746606c191573cc773f01172afbb52f33a9
Related: OS#5599
---
M doc/examples/mobile/default.cfg
M doc/examples/mobile/multi_ms.cfg
M src/host/layer23/configure.ac
M src/host/layer23/include/osmocom/bb/common/logging.h
M src/host/layer23/include/osmocom/bb/common/osmocom_data.h
A src/host/layer23/include/osmocom/bb/mobile/gapk_io.h
M src/host/layer23/include/osmocom/bb/mobile/settings.h
M src/host/layer23/src/common/logging.c
M src/host/layer23/src/mobile/Makefile.am
M src/host/layer23/src/mobile/app_mobile.c
A src/host/layer23/src/mobile/gapk_io.c
M src/host/layer23/src/mobile/gsm48_rr.c
M src/host/layer23/src/mobile/main.c
M src/host/layer23/src/mobile/settings.c
M src/host/layer23/src/mobile/voice.c
M src/host/layer23/src/mobile/vty_interface.c
16 files changed, 705 insertions(+), 40 deletions(-)