Add warning for unsupported DTX configurations

libosmo-abis do not consider DTX bits while processing TRAU frames. As I
do not have equipment to test it, I'm not sure if/how non-IP BTS will
work in case of DTX - warn users about it.

Change-Id: I94ee69cd309fc343a428ddc66942cd57f2a34c05
Related: OS#22
diff --git a/openbsc/src/libbsc/bsc_vty.c b/openbsc/src/libbsc/bsc_vty.c
index f4d47b4..b0f2f62 100644
--- a/openbsc/src/libbsc/bsc_vty.c
+++ b/openbsc/src/libbsc/bsc_vty.c
@@ -1748,7 +1748,9 @@
 	struct gsm_bts *bts = vty->index;
 
 	bts->dtxu = (argc > 0) ? GSM48_DTX_SHALL_BE_USED : GSM48_DTX_MAY_BE_USED;
-
+	if (!is_ipaccess_bts(bts))
+		vty_out(vty, "%% DTX enabled on non-IP BTS: this configuration "
+			"neither supported nor tested!%s", VTY_NEWLINE);
 	return CMD_SUCCESS;
 }
 
@@ -1771,7 +1773,9 @@
 	struct gsm_bts *bts = vty->index;
 
 	bts->dtxd = true;
-
+	if (!is_ipaccess_bts(bts))
+		vty_out(vty, "%% DTX enabled on non-IP BTS: this configuration "
+			"neither supported nor tested!%s", VTY_NEWLINE);
 	return CMD_SUCCESS;
 }