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