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*/