fix build error with stm32

hw/bsp/stm32f303disc/board_stm32f303disc.c

@@ -97,6 +97,12 @@ void board_led_control(bool state)
   HAL_GPIO_WritePin(GPIOE, GPIO_PIN_9, state);
 }
 
+uint32_t board_button_read(void)
+{
+  // TODO implement
+  return 0;
+}
+
 /*------------------------------------------------------------------*/
 /* TUSB HAL MILLISECOND
  *------------------------------------------------------------------*/

hw/bsp/stm32f407g_disc1/board_stm32f407g_disc1.c

@@ -91,6 +91,12 @@ void board_led_control(bool state)
   }
 }
 
+uint32_t board_button_read(void)
+{
+  // TODO implement
+  return 0;
+}
+
 
 /*------------------------------------------------------------------*/
 /* TUSB HAL MILLISECOND