boards/common/nucleo: use inverted gpio for user button

boards/common/nucleo/include/gpio_params.h

@@ -42,7 +42,8 @@ static const  saul_gpio_params_t saul_gpio_params[] =
     {
         .name = "Button(B1 User)",
         .pin  = BTN0_PIN,
-        .mode = BTN0_MODE
+        .mode = BTN0_MODE,
+        .flags = SAUL_GPIO_INVERTED
     },
 };