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/companion_radio/main.cpp

@@ -238,6 +238,8 @@ void setup() {
 #ifdef DISPLAY_CLASS
   ui_task.begin(disp, &sensors, the_mesh.getNodePrefs());  // still want to pass this in as dependency, as prefs might be moved
 #endif
+
+  board.bootComplete();
 }
 
 void loop() {

examples/kiss_modem/main.cpp

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

examples/simple_repeater/main.cpp

@@ -103,6 +103,8 @@ void setup() {
 #if ENABLE_ADVERT_ON_BOOT == 1
   the_mesh.sendSelfAdvertisement(16000, false);
 #endif
+
+  board.bootComplete();
 }
 
 void loop() {

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() {