AP_ICEngine: add singleton and remove unnecessary variable init

libraries/AP_ICEngine/AP_ICEngine.cpp

@@ -108,10 +108,14 @@ const AP_Param::GroupInfo AP_ICEngine::var_info[] = {
 
 // constructor
 AP_ICEngine::AP_ICEngine(const AP_RPM &_rpm) :
-    rpm(_rpm),
-    state(ICE_OFF)
+    rpm(_rpm)
 {
     AP_Param::setup_object_defaults(this, var_info);
+
+    if (_singleton != nullptr) {
+        AP_HAL::panic("AP_ICEngine must be singleton");
+    }
+    _singleton = this;
 }
 
 /*
@@ -289,3 +293,11 @@ bool AP_ICEngine::engine_control(float start_control, float cold_start, float he
     state = ICE_STARTING;
     return true;
 }
+
+// singleton instance. Should only ever be set in the constructor.
+AP_ICEngine *AP_ICEngine::_singleton;
+namespace AP {
+AP_ICEngine *ice() {
+        return AP_ICEngine::get_singleton();
+    }
+}

libraries/AP_ICEngine/AP_ICEngine.h

@@ -47,7 +47,11 @@ public:
     // handle DO_ENGINE_CONTROL messages via MAVLink or mission
     bool engine_control(float start_control, float cold_start, float height_delay);
     
+    static AP_ICEngine *get_singleton() { return _singleton; }
+
 private:
+    static AP_ICEngine *_singleton;
+
     const AP_RPM &rpm;
 
     enum ICE_State state;
@@ -95,3 +99,7 @@ private:
     bool height_pending:1;
 };
 
+
+namespace AP {
+    AP_ICEngine *ice();
+};