Manually add MDNS multicast Ethernet MACs to CYW43 (#1290)

SDK 1.5 changed the behavior of the underlying CYW43 blob, and it seems
to block MDNS multicast by default.  Manually add back the Ethernet MACs
used for MDNS multicast in IPV4 and IPV6.

Fixes #1267

Author: earlephilhower

libraries/lwIP_CYW43/src/utility/CYW43shim.cpp

@@ -68,10 +68,15 @@ bool CYW43::begin(const uint8_t* address, netif* netif) {
 
         // Not currently possible to hook up igmp_mac_filter and mld_mac_filter
         // TODO: implement igmp_mac_filter and mld_mac_filter
-#if LWIP_IPV6
-        // Implement cyw43_set_allmulti(_self, true) using exposed ioctl call
+        // Implement cyw43_set_allmulti(_self, true) using exposed ioctl call (may not be functional in SDK 1.5?)
         uint8_t allmulti_true[] = { 'a', 'l', 'l', 'm', 'u', 'l', 't', 'i', 0, 1, 0, 0, 0 };
         cyw43_ioctl(&cyw43_state, CYW43_IOCTL_SET_VAR, sizeof allmulti_true, allmulti_true, CYW43_ITF_STA);
+        // Add MDNS multicast MAC addresses manually, thanks Wikipedia!
+        uint8_t mdnsV4[] = {0x01, 0x00, 0x5E, 0x00, 0x00, 0xFB};
+        cyw43_wifi_update_multicast_filter(&cyw43_state, mdnsV4, true);
+#if LWIP_IPV6
+        uint8_t mdnsV6[] = {0x33, 0x33, 0x00, 0x00, 0x00, 0xFB};
+        cyw43_wifi_update_multicast_filter(&cyw43_state, mdnsV6, true);
 #endif
 
         if (_bssid[0] | _bssid[1] | _bssid[2] | _bssid[3] | _bssid[4] | _bssid[5]) {