mdns: add config option to use netif ext callback

e.g. if the calling stack should not invoke the mdns functions due
to high stack usage, disable the option and trigger it yourself.

Signed-off-by: Simon Goldschmidt <goldsimon@gmx.de>

src/include/lwip/apps/mdns_opts.h

@@ -59,6 +59,13 @@
 #define MDNS_MAX_SERVICES               1
 #endif
 
+/** MDNS_RESP_USENETIF_EXTCALLBACK==1: register an ext_callback on the netif
+ * to automatically restart probing/announcing on status or address change.
+ */
+#ifndef MDNS_RESP_USENETIF_EXTCALLBACK
+#define MDNS_RESP_USENETIF_EXTCALLBACK  LWIP_NETIF_EXT_STATUS_CALLBACK
+#endif
+
 /**
  * MDNS_DEBUG: Enable debugging for multicast DNS.
  */