SRV_Channel: prepare for reboot at the HAL rc output level

ensures that the 0s get to the hardware

significantly reduces the race conditions which might allow a thread to write after values are zeroed

libraries/SRV_Channel/SRV_Channel.h

@@ -603,6 +603,10 @@ public:
         return _singleton;
     }
 
+    // called once a reboot has been commanded: stop driving the
+    // outputs so the reset cannot truncate a pulse in flight
+    static void prepare_for_reboot();
+
     static void zero_rc_outputs();
 
     // initialize before any call to push

libraries/SRV_Channel/SRV_Channels.cpp

@@ -548,6 +548,12 @@ void SRV_Channels::push()
 #endif // HAL_ENABLE_DRONECAN_DRIVERS
 }
 
+void SRV_Channels::prepare_for_reboot()
+{
+    hal.rcout->prepare_for_reboot();
+}
+
+
 void SRV_Channels::zero_rc_outputs()
 {
     /* Send an invalid signal to the motors to prevent spinning due to