Add MainBoard::bootComplete() hook for boot-indicator LED feedback

Framework for upcoming variant-specific PRs that add LED feedback during boot. The hook gives users visual cues that the device is busy and
shouldn't be interacted with until startup completes.

examples/simple_room_server/main.cpp

@@ -80,6 +80,8 @@ void setup() {
 #if ENABLE_ADVERT_ON_BOOT == 1
   the_mesh.sendSelfAdvertisement(16000, false);
 #endif
+
+  board.bootComplete();
 }
 
 void loop() {