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/kiss_modem/main.cpp

@@ -119,6 +119,8 @@ void setup() {
   modem->setGetCurrentRssiCallback(onGetCurrentRssi);
   modem->setGetStatsCallback(onGetStats);
   modem->begin();
+
+  board.bootComplete();
 }
 
 void loop() {