AP_RTC: added get_semaphore()

libraries/AP_RTC/AP_RTC.cpp

@@ -63,6 +63,7 @@ void AP_RTC::set_utc_usec(uint64_t time_utc_usec, source_type type)
         // can't allow time to go backwards, ever
         return;
     }
+    WITH_SEMAPHORE(rsem);
 
     rtc_shift = tmp;
 

libraries/AP_RTC/AP_RTC.h

@@ -49,9 +49,15 @@ public:
         return _singleton;
     }
 
+    // allow threads to lock against RTC update
+    HAL_Semaphore &get_semaphore(void) {
+        return rsem;
+    }
+    
 private:
 
     static AP_RTC *_singleton;
+    HAL_Semaphore_Recursive rsem;
 
     source_type rtc_source_type = SOURCE_NONE;
     int64_t rtc_shift;