Changes

libosmo-trx: add TRXC/TRXD message codec

Introduce libosmo-trx: a shared implementation of the TRXC/TRXD
(OpenBTS-style TRX) protocol, to be used by osmo-trx, osmo-bts,
trxcon, and the upcoming C rewrite of fake_trx.

This initial version provides two I/O-free codec modules:

* trxd: TRXD PDU codec for BURST.ind and BURST.req
* trxc: verb-agnostic TRXC message codec (CMD/RSP/IND)

The codec logic is based on the existing implementations in
osmo-bts (f0ee51997470e1c3020ddb85082385ed43ba0c68).

Change-Id: I933fc417a67d0043f74b04626b7643c79e381492