feat: Add Russell, a board designed to go Up! on a balloon (#9079)

Hardware repository: https://github.com/Meshtastic-Malaysia/russell
- Designed to mount on an ER34615/IFR32700 cell
- RAK3172 STM32WLE5CCU6 MCU + integrated SX1262 LoRa
- CDtop CD-PA1010D GPS
- Bosch Sensortec BME280 sensor
- Consonance CN3158 LiFePO4 solar charger

Signed-off-by: Andrew Yong <me@ndoo.sg>

variants/stm32/russell/variant.h

@@ -0,0 +1,41 @@
+#ifndef _VARIANT_RUSSELL_
+#define _VARIANT_RUSSELL_
+
+#define USE_STM32WLx
+
+// I/O
+#define LED_PIN PA0 // Red LED
+#define LED_STATE_ON 1
+#define BUTTON_PIN PH3 // Shared with BOOT0
+#define BUTTON_NEED_PULLUP
+// Charger IC charge/standby pins are open-drain with no hardware pull-up:
+// Internal pull-up is needed on STM32 (TODO)
+// #define EXT_CHRG_DETECT PA5
+// #define EXT_PWR_DETECT PA4
+
+// Bosch Sensortec BME280
+#define HAS_SENSOR 1
+
+// CDtop CD-PA1010D
+#define ENABLE_HWSERIAL1
+#define PIN_SERIAL1_RX PB7
+#define PIN_SERIAL1_TX PB6
+#define HAS_GPS 1
+#define PIN_GPS_STANDBY PA15
+#define GPS_RX_PIN PB7
+#define GPS_TX_PIN PB6
+
+// LoRa
+/*
+ * RAK3172   (-20–85°C) -> No TCXO
+ * RAK3172-T (-40–85°C) -> 3.0V TCXO
+ * https://github.com/RAKWireless/RAK-STM32-RUI/blob/e5a28be8fab1a492bd9223dd425ca33a8a297d90/variants/WisDuo_RAK3172-T_Board/radio_conf.h#L91
+ */
+#define TCXO_OPTIONAL
+#define SX126X_DIO3_TCXO_VOLTAGE 3.0
+
+// Required to avoid Serial1 conflicts due to board definition here:
+// https://github.com/stm32duino/Arduino_Core_STM32/blob/main/variants/STM32WLxx/WL54CCU_WL55CCU_WLE4C(8-B-C)U_WLE5C(8-B-C)U/variant_RAK3172_MODULE.h
+#define RAK3172
+
+#endif