core: implement weak loop hook

Co-authored-by: Martino Facchin <m.facchin@arduino.cc>
Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>

Author: facchinm

cores/arduino/main.cpp

@@ -19,6 +19,10 @@ void start_static_threads();
 void __attribute__((weak)) initVariant(void) {
 }
 
+// This function can be overwriten by one library.
+void __attribute__((weak)) __loopHook(void) {
+}
+
 int main(void) {
 #if (DT_NODE_HAS_PROP(DT_PATH(zephyr_user), cdc_acm) &&                                            \
 	 (CONFIG_USB_CDC_ACM || CONFIG_USBD_CDC_ACM_CLASS))
@@ -39,6 +43,7 @@ int main(void) {
 	  // DT_NODE_HAS_PROP(DT_PATH(zephyr_user), serials)
     if (arduino::serialEventRun) arduino::serialEventRun();
 #endif
+		__loopHook();
 	}
 
 	return 0;