E1: change dummy fill frame generation method

In the absence of a proper TFO transform (TS 28.062 section
C3.2.1.1), OsmoMGW-E1 inserts constant fill frames when there is
no RTP input, or when RTP input fails conversion to TRAU-DL and
thus does not enqueue anything to the I.460 mux.  (We accept
TW-TS-001 and TW-TS-002 from RTP side, but bad frames cannot be
converted to TRAU-DL without a TFO transform.)

Prior to the present change, this idle fill of the DL was done
using fully hard-coded frames, i.e., the operation of converting
the desired fill to TRAU-DL bit format was done once externally
and the raw bit frames were hard-coded.  Change this approach:
use fill frames in RTP format provided by libosmocodec, and
execute osmo_rtp2trau() followed by osmo_trau_frame_encode() in
the fill frame output path just like we do for regular RTP to DL.

Depends: libosmocore.git I2c510ac62a0786c137115c45eee7a48b9736265f
Change-Id: I123f77295c56d61ff9aac5f5d64b25eca01e0d63
---
M TODO-RELEASE
M configure.ac
M src/libosmo-mgcp/Makefile.am
M src/libosmo-mgcp/mgcp_e1.c
M src/osmo-mgw/Makefile.am
5 files changed, 38 insertions(+), 157 deletions(-)