Make interface::CAN buildable on targets without DEVICE_CAN

This commit adds provisions to enable using interface::CAN on targets that don't have DEVICE_CAN set to 1 (ie: they don't normally have a CAN peripheral).

drivers/include/drivers/interfaces/InterfaceCAN.h

@@ -28,8 +28,10 @@
 
 namespace mbed {
 
+#ifndef FEATURE_EXPERIMENTAL_API
 // Forward declare CAN
 class CAN;
+#endif
 
 /** \defgroup drivers-public-api-can CAN
  * \ingroup drivers-public-api
@@ -171,6 +173,11 @@ using CAN = ::mbed::CAN;
 #endif
 
 } // namespace interface
+
+#if defined(FEATURE_EXPERIMENTAL_API) && !DEVICE_CAN
+using CAN = interface::CAN;
+#endif
+
 } // namespace mbed
 
 #endif /* MBED_INTERFACE_CAN_H_ */

hal/include/hal/can_helper.h

@@ -20,7 +20,7 @@
 #ifndef MBED_CAN_HELPER_H
 #define MBED_CAN_HELPER_H
 
-#if DEVICE_CAN
+#if DEVICE_CAN || FEATURE_EXPERIMENTAL_API
 
 #ifdef __cplusplus
 extern "C" {