Rename bootComplete() to onBootComplete() for naming consistency

Matches the existing event-style lifecycle hooks on MainBoard
(onBeforeTransmit, onAfterTransmit) per @liamcottle's review feedback.

examples/companion_radio/main.cpp

@@ -239,7 +239,7 @@ void setup() {
   ui_task.begin(disp, &sensors, the_mesh.getNodePrefs());  // still want to pass this in as dependency, as prefs might be moved
 #endif
 
-  board.bootComplete();
+  board.onBootComplete();
 }
 
 void loop() {

examples/kiss_modem/main.cpp

@@ -120,7 +120,7 @@ void setup() {
   modem->setGetStatsCallback(onGetStats);
   modem->begin();
 
-  board.bootComplete();
+  board.onBootComplete();
 }
 
 void loop() {

examples/simple_repeater/main.cpp

@@ -104,7 +104,7 @@ void setup() {
   the_mesh.sendSelfAdvertisement(16000, false);
 #endif
 
-  board.bootComplete();
+  board.onBootComplete();
 }
 
 void loop() {

examples/simple_room_server/main.cpp

@@ -81,7 +81,7 @@ void setup() {
   the_mesh.sendSelfAdvertisement(16000, false);
 #endif
 
-  board.bootComplete();
+  board.onBootComplete();
 }
 
 void loop() {