Changes

libosmotrau: add CSD RA2 functions

Rate adaption function RA2 for GSM CSD calls is defined in 3GPP TS
48.020 chapter 13.  While it is nothing but one special case of I.460
submultiplexing and we do have i460_mux infrastructure in Osmocom,
that fully-general I.460 infra is too heaviweight for CSD encoding
and decoding functions that need simple, single-input and single-output
RA2 packing and unpacking functions.

Right now ad hoc RA2 functions are used in the trau_rtp_conv module
and in OsmoBTS csd_v110; same functions will also be needed for the
upcoming RAA' unit test addition.  Add proper RA2 packing and unpacking
functions that can be used in all of these places.

Change-Id: I7b98b958651b3fc1a814d119d1b8644c91f98676

trau_rtp_conv: use new RA2 functions for CSD

osmo_trau2rtp() and osmo_rtp2trau() include CSD support; the RTP
format in CSD modes is RFC 4040 clearmode like the interface that
exists on the 64 kbit/s side of a traditional TRAU.  RA2 packing
and unpacking constitute required steps in these conversions -
use newly factored-out functions.

Change-Id: Ia157989579ced866b1e7abfc789d69ae489224e9