samr21-xpro/gpio: set LED/BUTTON gpios as inverted

This makes LED go off when set to 0 and button reads 1 when pressed.

boards/samr21-xpro/include/gpio_params.h

@@ -37,12 +37,14 @@ static const  saul_gpio_params_t saul_gpio_params[] =
     {
         .name = "LED(orange)",
         .pin = LED0_PIN,
-        .mode = GPIO_OUT
+        .mode = GPIO_OUT,
+        .flags = SAUL_GPIO_INVERTED,
     },
     {
         .name = "Button(SW0)",
         .pin  = BTN0_PIN,
-        .mode = BTN0_MODE
+        .mode = BTN0_MODE,
+        .flags = SAUL_GPIO_INVERTED,
     },
 };