Add initial SBc-AP support to osmo-cbc

This patch makes use of the newly introduced sbcap library, and
introduces new code (and extends existing one) to handle all the aspects
of MME peers talking SBc-AP and its underlaying SCTP connection.

This commit doesn't aim to implement all the SBc-AP features, but to
implement a minimal subset of features already available for CBSP in
osmo-cbc, in order to have similar support level for both 2G and 4G
networks.

Related: OS#4945
Change-Id: Ib278bc1d1a74459814016fef7a8fe21cc29d46c9
---
M configure.ac
M debian/control
M doc/examples/osmo-cbc/osmo-cbc.cfg
M include/osmocom/cbc/Makefile.am
M include/osmocom/cbc/cbc_data.h
M include/osmocom/cbc/internal.h
A include/osmocom/cbc/sbcap_msg.h
A include/osmocom/cbc/sbcap_server.h
M src/Makefile.am
M src/cbc_data.c
M src/cbc_main.c
M src/cbc_vty.c
M src/message_handling.c
A src/sbcap_msg.c
A src/sbcap_server.c
A src/sbcap_server_fsm.c
M src/smscb_message_fsm.c
M src/smscb_peer_fsm.c
18 files changed, 1,321 insertions(+), 54 deletions(-)