Rewrite statusled implementation.

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

fs/sonoff-basic.json

@@ -1,6 +1,7 @@
 {
   "name": "Sonoff Basic, S20, SV, RF, TH, Touch",
-  "status_led": 13,
+  "statusled": 13,
+  "statusled_invert": true,
   "channels": [
     { "led": -1, "relay": 12, "button": 0  }
   ]