[RBS2000] Actually activate the RBS2000 support

The previous patch introduces new code for the RBS2000 but didn't
actually activate/call it yet.

After this patch, you can see LAPD being established and OM2000 messages
will fall into abis_nm.c (which of course has no clue about them).
diff --git a/openbsc/src/bsc_hack.c b/openbsc/src/bsc_hack.c
index dacaad3..14e5d86 100644
--- a/openbsc/src/bsc_hack.c
+++ b/openbsc/src/bsc_hack.c
@@ -237,6 +237,7 @@
 	bts_model_unknown_init();
 	bts_model_bs11_init();
 	bts_model_nanobts_init();
+	bts_model_rbs2k_init();
 
 	e1inp_init();