AP_RPM: Add enabled() method to check if sensor is enabled

libraries/AP_RPM/AP_RPM.cpp

@@ -129,3 +129,18 @@ bool AP_RPM::healthy(uint8_t instance) const
     }
     return true;
 }
+
+/*
+  check if an instance is activated
+ */
+bool AP_RPM::enabled(uint8_t instance) const
+{
+    if (instance >= num_instances) {
+        return false;
+    }
+    // if no sensor type is selected, the sensor is not activated.
+    if (_type[instance] == RPM_TYPE_NONE) {
+        return false;
+    }
+    return true;
+}

libraries/AP_RPM/AP_RPM.h

@@ -77,6 +77,8 @@ public:
 
     bool healthy(uint8_t instance) const;
 
+    bool enabled(uint8_t instance) const;
+
 private:
     RPM_State state[RPM_MAX_INSTANCES];
     AP_RPM_Backend *drivers[RPM_MAX_INSTANCES];