Changes

E1: add support for AMR in TW-TS-006 enhanced RTP format

AMR speech codec on E1 BTS is currently possible at all only with
tw-e1abis-mgw and not with classic osmo-mgw.  This new E1 AMR MGW
support includes all 3 possible RTP formats from the beginning:
RFC 4867 BWE, RFC 4867 OA and TW-TS-006 OAX.

Because TW-TS-006 is non-standard outside of GSM networks
specifically built with Osmocom+ThemWi hybrid stack, use of this
RTP format has to be explicitly requested by the CN via BSSMAP
extension of TW-TS-003, which OsmoBSC already understands.

Add TW-TS-006 to the set of supported RTP extensions with E1 BTS,
joining already supported TW-TS-001 and TW-TS-002.

Change-Id: Icf859ca4975079c91a2f41190da0ce94aa686365