bsc_init: Allow DTXu and enable DTXd on RSL (experimental)

Allow the MS to use uplink discontinous transmission by
setting the right bit in the SystemInformation and set
DTXd/DTXu on the RSL channel commands.

This is configurable via dtx-used (0|1) on the network
level and still considered as experimental.
diff --git a/openbsc/src/bsc_init.c b/openbsc/src/bsc_init.c
index 2d92b42..4f7e29c 100644
--- a/openbsc/src/bsc_init.c
+++ b/openbsc/src/bsc_init.c
@@ -1153,8 +1153,14 @@
 	}
 
 	/* some defaults for our system information */
-	bts->si_common.cell_options.radio_link_timeout = 2; /* 12 */
-	bts->si_common.cell_options.dtx = 2; /* MS shall not use upplink DTX */
+	bts->si_common.cell_options.radio_link_timeout = 7; /* 12 */
+
+	/* allow/disallow DTXu */
+	if (bts->network->dtx_enabled)
+		bts->si_common.cell_options.dtx = 0;
+	else
+		bts->si_common.cell_options.dtx = 2;
+
 	bts->si_common.cell_options.pwrc = 0; /* PWRC not set */
 
 	bts->si_common.cell_sel_par.acs = 0;