AP_HAL_SITL: rename system_initialized() and add is_system_initialized()

libraries/AP_HAL_SITL/HAL_SITL_Class.cpp

@@ -211,7 +211,7 @@ void HAL_SITL::run(int argc, char * const argv[], Callbacks* callbacks) const
     fill_stack_nan();
 
     callbacks->setup();
-    scheduler->system_initialized();
+    scheduler->set_system_initialized();
 
 #ifndef HAL_NO_LOGGING
     if (getenv("SITL_WATCHDOG_RESET")) {

libraries/AP_HAL_SITL/Scheduler.cpp

@@ -143,7 +143,7 @@ void Scheduler::register_timer_failsafe(AP_HAL::Proc failsafe, uint32_t period_u
     _failsafe = failsafe;
 }
 
-void Scheduler::system_initialized() {
+void Scheduler::set_system_initialized() {
     if (_initialized) {
         AP_HAL::panic(
             "PANIC: scheduler system initialized called more than once");

libraries/AP_HAL_SITL/Scheduler.h

@@ -28,7 +28,8 @@ public:
     void register_timer_failsafe(AP_HAL::Proc, uint32_t period_us) override;
 
     bool in_main_thread() const override;
-    void system_initialized() override;
+    bool is_system_initialized() override { return _initialized; };
+    void set_system_initialized() override;
 
     void reboot(bool hold_in_bootloader) override;