Changes

trau: implement TFO frame decoding functions

TFO frames of 3GPP TS 28.062 are slightly modified TRAU-UL frames,
and thus require slightly modified encoding and decoding functions.
Adding TFO frame encoding and decoding functions to libosmotrau
will allow TFO applications (sw implementations of a TFO-capable
speech transcoder) to reuse those parts of TRAU frame encoding and
decoding that are common between regular TRAU frames and TFO.

Change-Id: Ia095598622645068d62301bc4bf7de9f68065e84