LCLS: add bts-loop variant

Add LCLS variant where the loop is closed on BTS level instead of
MGW. The main difference is the handling of connection-related
messages (we use IPA RSL instead of MGCP), the configuration and
correlation logic remains the same.

Change-Id: I7e8379f31037f2c48da69a01919701919a3066a2
Related: OS#3659
diff --git a/include/osmocom/bsc/bsc_msc_data.h b/include/osmocom/bsc/bsc_msc_data.h
index 32b161e..0c2094e 100644
--- a/include/osmocom/bsc/bsc_msc_data.h
+++ b/include/osmocom/bsc/bsc_msc_data.h
@@ -56,7 +56,7 @@
 enum bsc_lcls_mode {
 	BSC_LCLS_MODE_DISABLED,
 	BSC_LCLS_MODE_MGW_LOOP,
-	/* we may later introduce BTS_LOOP here: direct RTP between BTSs */
+	BSC_LCLS_MODE_BTS_LOOP,
 };
 
 extern const struct value_string bsc_lcls_mode_names[];