UDP for RAK4631 Eth Gw and the t-eth-elite. Solves #7149 (#7385)

* UDP for RAK4631 Eth Gw and the t-eth-elite. Also enable IP output on Portduino. Solves #7149

* Copilot suggestion

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix portduino build

* initialize local port

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/platform/nrf52/AsyncUDP.h

@@ -0,0 +1,57 @@
+#ifndef ASYNC_UDP_H
+#define ASYNC_UDP_H
+
+#include "concurrency/OSThread.h"
+#include <IPAddress.h>
+#include <Print.h>
+#include <RAK13800_W5100S.h>
+#include <cstdint>
+#include <functional>
+
+class AsyncUDPPacket;
+
+class AsyncUDP : public Print, private concurrency::OSThread
+{
+  public:
+    AsyncUDP();
+    explicit operator bool() const { return localPort != 0; }
+
+    bool listenMulticast(IPAddress multicastIP, uint16_t port, uint8_t ttl = 64);
+    bool writeTo(const uint8_t *data, size_t len, IPAddress ip, uint16_t port);
+
+    size_t write(uint8_t b) override;
+    size_t write(const uint8_t *data, size_t len) override;
+    void onPacket(const std::function<void(AsyncUDPPacket)> &callback);
+
+  private:
+    EthernetUDP udp;
+    uint16_t localPort;
+    std::function<void(AsyncUDPPacket)> _onPacket;
+    virtual int32_t runOnce() override;
+};
+
+class AsyncUDPPacket
+{
+  public:
+    AsyncUDPPacket(EthernetUDP &source);
+
+    IPAddress remoteIP();
+    uint16_t length();
+    const uint8_t *data();
+
+  private:
+    EthernetUDP &_udp;
+    IPAddress _remoteIP;
+    uint16_t _remotePort;
+    size_t _readLength = 0;
+
+    static constexpr size_t BUF_SIZE = 512;
+    uint8_t _buffer[BUF_SIZE];
+};
+
+inline bool isMulticast(const IPAddress &ip)
+{
+    return (ip[0] & 0xF0) == 0xE0;
+}
+
+#endif // ASYNC_UDP_H