types.h: Added NULL platform

Signed-off-by: Henry Bruce <henry.bruce@intel.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

api/mraa/types.h

@@ -50,6 +50,7 @@ typedef enum {
     // USB platform extenders start at 256
     MRAA_FTDI_FT4222 = 256,         /**< FTDI FT4222 USB to i2c bridge */
 
+    MRAA_NULL_PLATFORM = 98,        /**< Platform with no capabilities that hosts a sub platform  */
     MRAA_UNKNOWN_PLATFORM =
     99 /**< An unknown platform type, typically will load INTEL_GALILEO_GEN1 */
 } mraa_platform_t;