Fix native ethernet (#37)

* Replace the default Ethernet library with the NativeEthernet library

The Teensy 4.1 requires a direct replacement for the normal
Ethernet.h library by the NativeEthernet library.

NativeEthernet is a builtin library in platformio, therefore no extra
installations are required.

* fix de-referencing null pointer

The locator struct is not handed over, but the transport_* callbacks
depend on it. This result in a  NULL pointer in `transport->args`.

Author: fischejo

platform_code/arduino/native_ethernet/micro_ros_transport.cpp

@@ -2,7 +2,7 @@
 
 #include <micro_ros_platformio.h>
 
-#include <EthernetUdp.h>
+#include <NativeEthernetUdp.h>
 
 #include <uxr/client/util/time.h>
 #include <uxr/client/profile/transport/custom/custom_transport.h>
@@ -58,4 +58,4 @@ size_t platformio_transport_read(struct uxrCustomTransport * transport, uint8_t
   return (available > 0) ? available : 0;
 }
 
-}
+}

platform_code/arduino/native_ethernet/micro_ros_transport.h

@@ -1,4 +1,4 @@
-#include <Ethernet.h>
+#include <NativeEthernet.h>
 
 struct micro_ros_agent_locator {
 	IPAddress address;
@@ -17,10 +17,10 @@ static inline void set_microros_native_ethernet_transports(byte mac[], IPAddress
 
 	rmw_uros_set_custom_transport(
 		false,
-		NULL,
+		&locator,
 		platformio_transport_open,
 		platformio_transport_close,
 		platformio_transport_write,
 		platformio_transport_read
 	);
-}
+}