System.Platform boolean condition for RPi

In order to make an add-on setting visible only when running on a Raspberry Pi, it would be useful to have a boolean condition to test for that platform.

There are currently System.Platform.Android and System.Platform.ATV2 booleans so it seems logical that there should be one for Raspberry Pi? Perhaps called System.Platform.RPi?

I appreciate that there may be good reasons not to have this, but if not I would happily make a pull request if I could get some pointers. At first glance it seems I would only need to touch GUIInfoManager.cpp and GUIInfoManager.h?

I see there are already TARGET_RASPBERRY_PI ifdefs in various places.