common+trx: add rtp ecu-downstream vty option

Current osmo-bts-trx includes a provision for invoking ECUs from
libosmocodec in the UL path from the channel decoder to the RTP
output; no other models currently do likewise, but there is no
particular reason why this ECU invokation couldn't be moved into
model-independent code.

However, there are some network configurations in which this in-BTS
ECU invokation is undesirable, and the desire is to disable or
suppress it no matter which BTS model is used, rather than extend
the feature to BTS models which currently don't support it.
Usually the reason for wishing to suppress the ECU currently provided
by osmo-bts-trx is that another network element somewhere downstream
in RTP implements either a better ECU than what libosmocore provides
or a more complete Rx DTX handler of which the ECU is just one part.

The new rtp ecu-downstream vty option disables libosmocodec ECU
invokation in OsmoBTS.  The intended usage model is that this option
will be used together with rtp continuous-streaming, but in the
strict sense the two options are orthogonal.

Related: OS#5975
Change-Id: I0acca9c6d7da966a623287563e0789db9e0fae8e
---
M include/osmo-bts/bts.h
M src/common/vty.c
M src/osmo-bts-trx/l1_if.c
M tests/osmo-bts.vty
4 files changed, 70 insertions(+), 3 deletions(-)