Add WiFi companion support for Pico W

SerialWifiInterface has no ESP32-specific code, so move it to
helpers/wifi and reuse it on RP2040. Guard the ESP32-only WiFi
event/auto-reconnect calls and poll link state on RP2040 instead.

src/helpers/wifi/SerialWifiInterface.cpp

@@ -0,0 +1,172 @@
+#include "SerialWifiInterface.h"
+#include <WiFi.h>
+
+void SerialWifiInterface::begin(int port) {
+  // wifi setup is handled outside of this class, only starts the server
+  server.begin(port);
+}
+
+// ---------- public methods
+void SerialWifiInterface::enable() { 
+  if (_isEnabled) return;
+
+  _isEnabled = true;
+  clearBuffers();
+}
+
+void SerialWifiInterface::disable() {
+  _isEnabled = false;
+}
+
+size_t SerialWifiInterface::writeFrame(const uint8_t src[], size_t len) {
+  if (len > MAX_FRAME_SIZE) {
+    WIFI_DEBUG_PRINTLN("writeFrame(), frame too big, len=%d\n", len);
+    return 0;
+  }
+
+  if (deviceConnected && len > 0) {
+    if (send_queue_len >= FRAME_QUEUE_SIZE) {
+      WIFI_DEBUG_PRINTLN("writeFrame(), send_queue is full!");
+      return 0;
+    }
+
+    send_queue[send_queue_len].len = len;  // add to send queue
+    memcpy(send_queue[send_queue_len].buf, src, len);
+    send_queue_len++;
+
+    return len;
+  }
+  return 0;
+}
+
+bool SerialWifiInterface::isWriteBusy() const {
+  return false;
+}
+
+bool SerialWifiInterface::hasReceivedFrameHeader() {
+  return received_frame_header.type != 0 && received_frame_header.length != 0;
+}
+
+void SerialWifiInterface::resetReceivedFrameHeader() {
+  received_frame_header.type = 0;
+  received_frame_header.length = 0;
+}
+
+size_t SerialWifiInterface::checkRecvFrame(uint8_t dest[]) {
+  // check if new client connected
+  auto newClient = server.available();
+  if (newClient) {
+
+    // disconnect existing client
+    deviceConnected = false;
+    client.stop();
+
+    // switch active connection to new client
+    client = newClient;
+
+    // forget received frame header
+    resetReceivedFrameHeader();
+    
+  }
+
+  if (client.connected()) {
+    if (!deviceConnected) {
+      WIFI_DEBUG_PRINTLN("Got connection");
+      deviceConnected = true;
+    }
+  } else {
+    if (deviceConnected) {
+      deviceConnected = false;
+      WIFI_DEBUG_PRINTLN("Disconnected");
+    }
+  }
+
+  if (deviceConnected) {
+    if (send_queue_len > 0) {   // first, check send queue
+      
+      _last_write = millis();
+      int len = send_queue[0].len;
+
+      uint8_t pkt[3+len]; // use same header as serial interface so client can delimit frames
+      pkt[0] = '>';
+      pkt[1] = (len & 0xFF);  // LSB
+      pkt[2] = (len >> 8);    // MSB
+      memcpy(&pkt[3], send_queue[0].buf, send_queue[0].len);
+      client.write(pkt, 3 + len);
+      send_queue_len--;
+      for (int i = 0; i < send_queue_len; i++) {   // delete top item from queue
+        send_queue[i] = send_queue[i + 1];
+      }
+    } else {
+
+      // check if we are waiting for a frame header
+      if(!hasReceivedFrameHeader()){
+
+        // make sure we have received enough bytes for a frame header
+        // 3 bytes frame header = (1 byte frame type) + (2 bytes frame length as unsigned 16-bit little endian)
+        int frame_header_length = 3;
+        if(client.available() >= frame_header_length){
+
+          // read frame header
+          client.readBytes(&received_frame_header.type, 1);
+          client.readBytes((uint8_t*)&received_frame_header.length, 2);
+
+        }
+
+      }
+
+      // check if we have received a frame header
+      if(hasReceivedFrameHeader()){
+
+        // make sure we have received enough bytes for the required frame length
+        int available = client.available();
+        int frame_type = received_frame_header.type;
+        int frame_length = received_frame_header.length;
+        if(frame_length > available){
+          WIFI_DEBUG_PRINTLN("Waiting for %d more bytes", frame_length - available);
+          return 0;
+        }
+
+        // skip frames that are larger than MAX_FRAME_SIZE
+        if(frame_length > MAX_FRAME_SIZE){
+          WIFI_DEBUG_PRINTLN("Skipping frame: length=%d is larger than MAX_FRAME_SIZE=%d", frame_length, MAX_FRAME_SIZE);
+          while(frame_length > 0){
+            uint8_t skip[1];
+            int skipped = client.read(skip, 1);
+            frame_length -= skipped;
+          }
+          resetReceivedFrameHeader();
+          return 0;
+        }
+
+        // skip frames that are not expected type
+        // '<' is 0x3c which indicates a frame sent from app to radio
+        if(frame_type != '<'){
+          WIFI_DEBUG_PRINTLN("Skipping frame: type=0x%x is unexpected", frame_type);
+          while(frame_length > 0){
+            uint8_t skip[1];
+            int skipped = client.read(skip, 1);
+            frame_length -= skipped;
+          }
+          resetReceivedFrameHeader();
+          return 0;
+        }
+
+        // read frame data to provided buffer
+        client.readBytes(dest, frame_length);
+
+        // ready for next frame
+        resetReceivedFrameHeader();
+        return frame_length;
+
+      }
+      
+    }
+  }
+
+  return 0;
+}
+
+bool SerialWifiInterface::isConnected() const {
+  return deviceConnected;  //pServer != NULL && pServer->getConnectedCount() > 0;
+}