bsc: Add Lb interface support

This introduces the Lb interface stack, which allows BSC_Tests.ttcn
to emulate a SMLC towards the BSC.

In accordance with https://osmocom.org/projects/cellular-infrastructure/wiki/Point_Codes
we use 0.23.6 as point code for emulating the SMLC.

Change-Id: I854618cc08de1a716784f52542a4df3c7f7ad900
3 files changed