Changes

trau: implement TFO frame encoding function

Most differences between regular TRAU frames of TS 48.060 & 48.061
and TFO frames of TS 28.062 matter only in the frame decoding
direction.  In the encoding direction, regular osmo_trau_frame_encode()
would have been sufficient for both frame types if it weren't for
the quirk of setting C1..C5 bits.  osmo_trau_frame_encode() sets
these bits per its own mind for most frame types, ignoring the bits
passed by the user in fr->c_bits[].  This API behavior cannot be
changed without breaking existing users, but it is bad for TFO.

Solution: implement new osmo_trau_frame_encode_tfo() function
for TFO applications.  As an additional benefit, the output buffer
space requirement is reduced because there is no possibility of
output frame extension for timing alignment.

Change-Id: I6e0ab4e5115f0aa6979ce93955b0e95b09041091