Added bridgin support for GSM and SIP

Added bridgin support for GSM and SIP

The dependency on mISDN (loopback interface) is completely removed
from GSM and SIP interfaces.

The built in bridge of LCR now forwards audio data between these
interface instances or between these instances and other instances.

Additionally both GSM BS and SIP support direct forwarding of RTP
traffic between  other SIP endpoint and OpenBSC, so no traffic is
forwarded by the LCR itself. This is done by forwarding RTP peer
informations between these interface instances.