LOGGING: make sure to make the 'logging filter' compatible with vty log cfg

Recent libosmocore introdues a way how log targets can be configured from
the VTY.

This commit makes the 'log filter (imsi|nsvc|bvc)' compatible with it.
diff --git a/openbsc/src/bsc_vty.c b/openbsc/src/bsc_vty.c
index 55a667c..6e07a6b 100644
--- a/openbsc/src/bsc_vty.c
+++ b/openbsc/src/bsc_vty.c
@@ -2504,18 +2504,16 @@
 	LOGGING_STR FILTER_STR
       "Filter log messages by IMSI\n" "IMSI to be used as filter\n")
 {
-	struct telnet_connection *conn;
+	struct log_target *tgt = osmo_log_vty2tgt(vty);
 
-	conn = (struct telnet_connection *) vty->priv;
-	if (!conn->dbg) {
-		vty_out(vty, "Logging was not enabled.%s", VTY_NEWLINE);
+	if (!tgt)
 		return CMD_WARNING;
-	}
 
-	log_set_imsi_filter(conn->dbg, argv[0]);
+	log_set_imsi_filter(tgt, argv[0]);
 	return CMD_SUCCESS;
 }
 
+
 DEFUN(drop_bts,
       drop_bts_cmd,
       "drop bts connection <0-65535> (oml|rsl)",
@@ -2635,6 +2633,7 @@
 	install_element_ve(&show_paging_cmd);
 
 	logging_vty_add_cmds();
+	install_element(CFG_LOG_NODE, &logging_fltr_imsi_cmd);
 
 	install_element(CONFIG_NODE, &cfg_net_cmd);
 	install_node(&net_node, config_write_net);