[rbp] Fix new settings scheme for RPi and disable unimplemented options

The Pi has both TARGET_LINUX and TARGET_RASPBERRY_PI defined, so the ordering of the checks in settings.c needs to be changed or rbp.xml is not used.
I've made rbp.xml be included after linux.xml, following how it's done for TARGET_DARWIN.
I've also been through the settings carefully and have made any settings that are not implemented on Pi be not-visible.

If and when support is added for probing capabilities of the media player then the hiding of unsupported options can be removed.

Thanks to montellese for making this configuration so simple.