System.Platform boolean condition for RPi

This adds a System.Platform.Linux.RaspberryPi boolean condition.

This is primarily to allow add-on settings to be visible only when running on a Raspberry Pi, using
`<setting ... enable="System.Platform.Linux.RaspberryPi"/>`.

There are currently System.Platform.Android and System.Platform.ATV2 booleans so it seems logical that there should be one for Raspberry Pi too.