The aim is to add configurable audio loopback to mobile. An existing patch on a
branch from fixeria [1] adds the audio config section. Add a reduced version of
this audio config to be compatible with the future merge.
Add the audio loopback setting, so far without functionality.
Subsequent patch adds the actual loopback.
[1] osmocom-bb branch fixeria/audio,
patch "mobile/vty_interface.c: add new 'audio' section"
Change-id I62cd5ef22ca2290fcafe65c78537ddbcb39fb8c6
Change-Id: Ie03e4a6c6f81ea3925266dd22e87506d722a6e1a