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 libosmogapk [1][2], which in its turn
uses the ALSA sound system for the playback and capture.  This
is a new optional dependency of mobile, which is automatically
picked up if available during the build configuration.  Whether
to depend on it or not can be controlled using '--with-gapk-io'.

The API offered by libosmogapk implies to use the processing chains,
which generally consist of a source block, several processing blocks,
and a sink block.  The mobile app implements the following chains:

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

both taking/storing TCH frames from/to the following two buffers:

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

The buffers are served by a new function gapk_io_dequeue().

[1] https://gitea.osmocom.org/osmocom/gapk/
[1] https://osmocom.org/projects/gapk

Change-Id: Ib86b0746606c191573cc773f01172afbb52f33a9
Related: OS#5599
---
M contrib/jenkins.sh
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
M src/host/layer23/include/osmocom/bb/mobile/Makefile.am
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
18 files changed, 761 insertions(+), 8 deletions(-)