[Gratuitous ARP] Only send ARP packets when IF is up

See this PR: xoseperez/espurna#1877  and discussion here: esp8266/Arduino#5998 (comment)


Fixes letscontrolit#2374

src/Networking.ino

@@ -16,6 +16,10 @@
  # endif // if !defined(ARDUINO_ESP8266_RELEASE_2_4_0) && !defined(ARDUINO_ESP8266_RELEASE_2_3_0)
 #endif // ifdef ESP8266
 
+#ifdef ESP32
+#define SUPPORT_ARP
+#endif
+
 #ifdef SUPPORT_ARP
 # include <lwip/etharp.h>
 #endif // ifdef SUPPORT_ARP
@@ -918,7 +922,11 @@ void sendGratuitousARP() {
  netif *n = netif_list;
 
  while (n) {
- etharp_gratuitous(n);
+ if ((n->hwaddr_len == ETH_HWADDR_LEN) && 
+ (n->flags & NETIF_FLAG_ETHARP) && 
+ ((n->flags & NETIF_FLAG_LINK_UP) || (n->flags & NETIF_FLAG_UP))) {
+ etharp_gratuitous(n);
+ }
  n = n->next;
  }
  STOP_TIMER(GRAT_ARP_STATS);