mdns: move MDNS_PROBE_DELAY_MS to mdns_opts.h to allow customisation

In some noisy WiFi environment, it may be necessary to increase this value to
300ms to accomodate WiFi latencies which may result in less than the required
250ms between two probe frames received by the Apple BCT application.

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

src/apps/mdns/mdns.c

@@ -126,7 +126,6 @@ static mdns_name_result_cb_t mdns_name_result_cb;
                              MDNS_RESPONSE_TC_DELAY_MIN) + MDNS_RESPONSE_TC_DELAY_MIN)
 
 /** Probing & announcing defines */
-#define MDNS_PROBE_DELAY_MS       250
 #define MDNS_PROBE_COUNT          3
 #ifdef LWIP_RAND
 /* first probe timeout SHOULD be random 0-250 ms*/

src/include/lwip/apps/mdns_opts.h

@@ -60,6 +60,14 @@
 #define MDNS_MAX_SERVICES               1
 #endif
 
+/** The minimum delay between probes in ms. RFC 6762 require 250ms.
+ * In noisy WiFi environment, adding 30-50ms to this value help a lot for
+ * a successful Apple BCT tests.
+ */
+#ifndef MDNS_PROBE_DELAY_MS
+#define MDNS_PROBE_DELAY_MS           250
+#endif
+
 /** The maximum number of received packets stored in chained list of known
  * answers for pending truncated questions. This value define the size of
  * the MDNS_PKTS mempool.