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.

src/MeshCore.h

@@ -52,6 +52,10 @@ public:
   virtual void onAfterTransmit() { }
   virtual void reboot() = 0;
   virtual void powerOff() { /* no op */ }
+  // Called by example setup() functions to signal that boot is complete.
+  // Boards may override to stop a boot-indicator LED sequence or similar.
+  // Default no-op: boards that don't care need not implement anything.
+  virtual void bootComplete() { /* no op */ }
   virtual void sleep(uint32_t secs)  { /* no op */ }
   virtual uint32_t getGpio() { return 0; }
   virtual void setGpio(uint32_t values) {}