Add shared board class for ExpressLRS ESP32 TX modules

ExpressLRS transmitter modules share a common shape: powered from the
handset bay or USB rather than a battery, a fan over the amplifier, and an
ESP8285 "backpack" on a second UART that MeshCore has no use for.

ELRSTxBoard handles those from optional defines (PIN_FAN_EN,
PIN_BACKPACK_EN, PIN_BACKPACK_BOOT, MANUFACTURER_NAME) so a variant only
has to supply pin numbers. It also returns DIO0 from getIRQGpio(), since
the ESP32Board default is the SX126x pin.

src/helpers/esp32/ELRSTxBoard.h

@@ -0,0 +1,55 @@
+#pragma once
+
+#include <Arduino.h>
+#include <helpers/ESP32Board.h>
+
+// The common shape of an ExpressLRS ESP32 transmitter module: powered from the
+// JR bay or USB rather than a battery, a fan over the amplifier, and an
+// ESP8285 "backpack" hanging off a second UART.
+//
+// A variant supplies whichever of these its ExpressLRS hardware.json lists:
+//
+//   MANUFACTURER_NAME   name reported to clients
+//   PIN_FAN_EN          hardware.json "misc_fan_en"     (HIGH = on)
+//   PIN_BACKPACK_EN     hardware.json "backpack_en"     (HIGH = enabled)
+//   PIN_BACKPACK_BOOT   hardware.json "backpack_boot"
+//
+// There is no battery divider on these boards, and ESP32Board already returns
+// 0 from getBattMilliVolts() when PIN_VBAT_READ is undefined, so nothing here
+// needs to override it.
+
+#ifndef MANUFACTURER_NAME
+  #define MANUFACTURER_NAME "ExpressLRS TX"
+#endif
+
+class ELRSTxBoard : public ESP32Board {
+public:
+  void begin() {
+    ESP32Board::begin();
+
+  #ifdef PIN_BACKPACK_EN
+    // MeshCore has no use for the backpack. Hold it off so it cannot chatter
+    // on the shared UART.
+    pinMode(PIN_BACKPACK_EN, OUTPUT);
+    digitalWrite(PIN_BACKPACK_EN, LOW);
+  #endif
+  #ifdef PIN_BACKPACK_BOOT
+    pinMode(PIN_BACKPACK_BOOT, INPUT);   // usually a strapping pin, leave it
+  #endif
+
+  #ifdef PIN_FAN_EN
+    // ExpressLRS only spins the fan above 250 mW, but MeshCore transmits for
+    // far longer than an ExpressLRS packet, so just leave it running.
+    pinMode(PIN_FAN_EN, OUTPUT);
+    digitalWrite(PIN_FAN_EN, HIGH);
+  #endif
+  }
+
+  const char* getManufacturerName() const override {
+    return MANUFACTURER_NAME;
+  }
+
+  uint32_t getIRQGpio() override {
+    return P_LORA_DIO_0;   // SX127x signals RxDone/TxDone on DIO0
+  }
+};