Rewrite statusled implementation.

- Read it now from app.config JSON file.
- Allow for inverted status leds (Sonoff et al)
- Clean up unit tests.

include/main.h

@@ -6,7 +6,6 @@
 
 #define CHANNEL_CHANGE_COOLDOWN_SECONDS 0.250
 #define CHANNEL_MAX 16
-#define GREEN_LED_GPIO 0
 #define GPIO_INVALID 255
 #define GPIO_MIN 0
 #define GPIO_MAX 16
@@ -19,7 +18,8 @@ struct channel_t {
   double button_last_change;
 };
 
-void led_status_blink();
+void statusled_blink();
+void statusled_init(uint8_t gpio, bool state_off);
 
 bool channel_init(const char *fn);
 uint8_t channel_gpio_by_idx(int idx);