feat(xiao): expose onboard BOOT0 user button

Author: cumin777

variants/STM32C5xx/C593C(E-G)(T-U)_C5A3CG(T-U)/variant_XIAO_STM32C5.cpp

@@ -27,7 +27,8 @@ const PinName digitalPin[] = {
   PB_3,  // Onboard IMU / I2C2_SCL
   PB_4,  // Onboard IMU / I2C2_SDA
   PC_13, // Onboard IMU / INT1
-  PA_8   // Onboard IMU heater / TIM1_CH1
+  PA_8,  // Onboard IMU heater / TIM1_CH1
+  PH_2   // BOOT0 / user button (active low)
 };
 
 const pin_size_t analogInputPin[] = {

variants/STM32C5xx/C593C(E-G)(T-U)_C5A3CG(T-U)/variant_XIAO_STM32C5.h

@@ -28,19 +28,27 @@
 #define PIN_IMU_SDA             18  // PB4 / I2C2_SDA
 #define PIN_IMU_INT1            19  // PC13 / LSM6DS3TR-C INT1
 #define PIN_IMU_HEATER          20  // PA8 / TIM1_CH1
+#define PH2                     21  // BOOT0 / user button
 
 /* Alternate functions used by the default board peripherals. */
 #define PA9_ALT1                (PA9 | ALT1)
 #define PA10_ALT1               (PA10 | ALT1)
 #define PB15_ALT2               (PB15 | ALT2)
 
-#define NUM_DIGITAL_PINS        21
+#define NUM_DIGITAL_PINS        22
 #define NUM_ANALOG_INPUTS       4
 
 #ifndef LED_BUILTIN
   #define LED_BUILTIN           PB12
 #endif
 
+// On-board user button. The physical button is active low and is also
+// connected to the MCU BOOT0/PH2 pin.
+#define B1_BTN                  PH2
+#ifndef USER_BTN
+  #define USER_BTN              B1_BTN
+#endif
+
 /* SPI3: D8/SCK (PE2), D9/MISO (PB0), D10/MOSI (PB15).
  * No board-default chip-select is assigned; sketches control device CS. */
 #define PIN_SPI_SS              NUM_DIGITAL_PINS