AP_HAL: added set_esc_scaling() RCOutput function

this will be used to scale outputs for uavcan ESCs based on throttle
range

libraries/AP_HAL/RCOutput.h

@@ -71,6 +71,14 @@ public:
       force the safety switch off, enabling PWM output from the IO board
      */
     virtual void     force_safety_off(void) {}
+
+    /*
+      setup scaling of ESC output for ESCs that can output a
+      percentage of power (such as UAVCAN ESCs). The values are in
+      microseconds, and represent minimum and maximum PWM values which
+      will be used to convert channel writes into a percentage
+     */
+    virtual void     set_esc_scaling(uint16_t min_pwm, uint16_t max_pwm) {}
 };
 
 #endif // __AP_HAL_RC_OUTPUT_H__