Add basic SI2quater support

* support for sending arbitrary static SI2quater.
* vty interface for neightbor EARFCNs specific to SI2quater.
* dynamic generation of SI2quater messages.
* unit test for SI2quater messages.

Fixes: OS#1630
diff --git a/openbsc/src/libbsc/bsc_init.c b/openbsc/src/libbsc/bsc_init.c
index fd8dd66..fea6562 100644
--- a/openbsc/src/libbsc/bsc_init.c
+++ b/openbsc/src/libbsc/bsc_init.c
@@ -192,9 +192,9 @@
 
 	return 0;
 err_out:
-	LOGP(DRR, LOGL_ERROR, "Cannot generate SI%s for BTS %u, most likely "
-		"a problem with neighbor cell list generation\n",
-		get_value_string(osmo_sitype_strs, i), bts->nr);
+	LOGP(DRR, LOGL_ERROR, "Cannot generate SI%s for BTS %u: error <%s>,"
+	     "most likely a problem with neighbor cell list generation\n",
+	     get_value_string(osmo_sitype_strs, i), bts->nr, strerror(-rc));
 	return rc;
 }