Ethernet: align ctor to WiFi

Author: pennam

libraries/Ethernet/src/Ethernet.cpp

@@ -3,6 +3,9 @@
 #define SSID_MAX_LENGTH 32
 
 int arduino::EthernetClass::begin(uint8_t *mac, unsigned long timeout, unsigned long responseTimeout) {
+  if (eth_if == nullptr) {
+    return 0;
+  }
   eth_if->set_dhcp(true);
   return _begin(mac, timeout, responseTimeout);
 }
@@ -94,4 +97,4 @@ void arduino::EthernetClass::MACAddress(uint8_t *mac_address)
   macAddress(mac_address);
 }
 
-arduino::EthernetClass Ethernet;
+arduino::EthernetClass Ethernet(static_cast<EthernetInterface*>(EthInterface::get_default_instance()));

libraries/Ethernet/src/Ethernet.h

@@ -60,7 +60,6 @@ class EthernetClass : public MbedSocketClass {
   // Returns 0 if the DHCP configuration failed, and 1 if it succeeded
   EthernetClass(EthernetInterface *_if)
     : eth_if(_if){};
-  EthernetClass(){};
 
   int begin(uint8_t *mac = nullptr, unsigned long timeout = 60000, unsigned long responseTimeout = 4000);
   EthernetLinkStatus linkStatus();
@@ -116,8 +115,7 @@ class EthernetClass : public MbedSocketClass {
   int _begin(uint8_t *mac, unsigned long timeout, unsigned long responseTimeout);
 
   volatile EthernetLinkStatus _currentNetworkStatus = Unknown;
-  EthernetInterface net;
-  EthernetInterface *eth_if = &net;
+  EthernetInterface *eth_if = nullptr;
   arduino::IPAddress ipAddressFromSocketAddress(SocketAddress socketAddress);
 };