raspberrypi: new Nerves platform
rpi: enable unionfs

This doesn't work unless you pre-download the unionfs patch to the
download directory since buildroot gets confused with the kernel being
downloaded via git and the patches being downloaded via http.
Update rpi to common busybox
Enable mtools for RPi and add conf file

The conf file works around mtools complaining when sector sizes aren't
too its liking.
Cache buildroot download to reduce rebuild time
rpi: fix unionfs config