AP_HAL: make erpm accessible from RCOutput

give access to erpm error rate
add bi-directional dshot mask modifier

libraries/AP_HAL/RCOutput.h

@@ -122,6 +122,12 @@ public:
      */
     virtual float    scale_esc_to_unity(uint16_t pwm) { return 0; }
 
+    /*
+      return the erpm and error rate for a channel if available
+     */
+    virtual uint16_t get_erpm(uint8_t chan) const { return 0; }
+    virtual float get_erpm_error_rate(uint8_t chan) const { return 100.0f; }
+
     /*
       enable PX4IO SBUS out at the given rate
      */
@@ -199,6 +205,12 @@ public:
      */
     virtual void set_telem_request_mask(uint16_t mask) {}
 
+    /*
+      enable bi-directional telemetry request for a mask of channels. This is used
+      with DShot to get telemetry feedback
+     */
+    virtual void set_bidir_dshot_mask(uint16_t mask) {}
+
     /*
       setup serial led output for a given channel number, with
       the given max number of LEDs in the chain.