update AP_Mount mount_open servo output to work for all backends

The SRV_Channel library had a mount_open and mount2_open functions but they were only used by the Mount_Servo backend.

Moved the little bit of code that controls to servo into a function in AP_Mount_Backend and then called it from every backend as part of the update() call.

libraries/AP_Mount/AP_Mount_Backend.cpp

@@ -118,6 +118,34 @@ void AP_Mount_Backend::set_rate_target(float roll_degs, float pitch_degs, float
     }
 }
 
+void AP_Mount_Backend::control_mount_open_servo() {
+    // check if the open function is assigned to a servo channel.
+    uint8_t open_chan;
+    if (!SRV_Channels::find_channel(SRV_Channel::k_mount_open, open_chan) &&
+        !SRV_Channels::find_channel(SRV_Channel::k_mount2_open, open_chan)) {
+        return; // open function not assigned, do nothing.
+    }
+
+    // get the configured PWM values for open and closed states.
+    SRV_Channel *channel = SRV_Channels::srv_channel(open_chan);
+    if (!channel) {
+        return; // channel not found.
+    }
+    uint16_t pwm_open = channel->get_output_max();
+    uint16_t pwm_closed = channel->get_output_min();
+
+    // determine the desired open state based on the current mode.
+    bool mount_open = true;
+    if (get_mode() == MAV_MOUNT_MODE_RETRACT) {
+        mount_open = false;
+    }
+
+    // move the servo to the appropriate position.
+    uint16_t target_pwm = mount_open ? pwm_open : pwm_closed;
+    SRV_Channels::move_servo(SRV_Channel::Aux_servo_function_t(open_chan), 
+                              target_pwm, pwm_closed, pwm_open);
+}
+
 // set_roi_target - sets target location that mount should attempt to point towards
 void AP_Mount_Backend::set_roi_target(const Location &target_loc)
 {

libraries/AP_Mount/AP_Mount_Backend.h

@@ -293,6 +293,9 @@ protected:
     // sent warning to GCS
     void send_warning_to_GCS(const char* warning_str);
 
+    // control the open/close servo based on current mode and parameters.
+    void control_mount_open_servo();
+
     AP_Mount    &_frontend; // reference to the front end which holds parameters
     AP_Mount_Params &_params; // parameters for this backend
     uint8_t     _instance;  // this instance's number

libraries/AP_Mount/AP_Mount_Servo.cpp

@@ -30,6 +30,9 @@ void AP_Mount_Servo::update()
     // change to RC_TARGETING mode if RC input has changed
     set_rctargeting_on_rcinput_change();
 
+    // control the open/close servo.
+    control_mount_open_servo();
+
     auto mount_mode = get_mode();
     switch (mount_mode) {
         // move mount to a "retracted position" or to a position where a fourth servo can retract the entire mount into the fuselage
@@ -109,9 +112,6 @@ void AP_Mount_Servo::update()
             break;
     }
 
-    // move mount to a "retracted position" into the fuselage with a fourth servo
-    const bool mount_open = (mount_mode == MAV_MOUNT_MODE_RETRACT) ? 0 : 1;
-    move_servo(_open_idx, mount_open, 0, 1);
 
     // write the results to the servos
     move_servo(_roll_idx, degrees(_angle_bf_output_rad.x)*10, _params.roll_angle_min*10, _params.roll_angle_max*10);