AP_HAL: added set_safety_pwm() API

libraries/AP_HAL/RCOutput.h

@@ -45,6 +45,12 @@ public:
      * array of channels. */
     virtual uint16_t read(uint8_t ch) = 0;
     virtual void     read(uint16_t* period_us, uint8_t len) = 0;
+
+    /*
+      set PWM to send to a set of channels when the safety switch is
+      in the safe state
+     */
+    virtual void     set_safety_pwm(uint32_t chmask, uint16_t period_us) {}
 };
 
 #endif // __AP_HAL_RC_OUTPUT_H__