AP_HAL: added in_expected_delay()

allows for error message suppression when delays are expected

libraries/AP_HAL/Scheduler.h

@@ -37,6 +37,12 @@ public:
      */
     virtual void     expect_delay_ms(uint32_t ms) { }
 
+    /*
+      return true if we are in a period of expected delay. This can be
+      used to suppress error messages
+     */
+    virtual bool     in_expected_delay(void) const { return false; }
+
     /*
       end the priority boost from delay_microseconds_boost()
      */