Move SI-related defines

* move SI2quater related defines to shared header
* add define from OsmoBTS which checks for presence of a given SI
  message in gsm_bts struct. Rename it to avoid conflicts with OsmoBTS
  code and to match naming conventions of similar macros.

Change-Id: I11432c93c772d1ead6d45a7bb0f1d13d492c82f1
Related: OS#1660
diff --git a/openbsc/src/libbsc/bsc_init.c b/openbsc/src/libbsc/bsc_init.c
index e5226a9..25f3fdc 100644
--- a/openbsc/src/libbsc/bsc_init.c
+++ b/openbsc/src/libbsc/bsc_init.c
@@ -186,7 +186,7 @@
 
 	for (n = 0; n < n_si; n++) {
 		i = gen_si[n];
-		if (!(bts->si_valid & (1 << i)))
+		if (!GSM_BTS_HAS_SI(bts, i))
 			continue;
 		rc = rsl_si(trx, i, si_len[i]);
 		if (rc < 0)