[WIP] Add raspberry3 target
I was able to build an image, but I haven't gotten it to boot yet.
- Requires vmdebootstrap to have this patch applied: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=845439
- Patch for flash-kernel submitted here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=869488
- It's possible to just install raspi3-firmware on arm64, in which case it would it runs a kernel postinst hook (https://anonscm.debian.org/cgit/pkg-raspi/raspi3-firmware.git/tree/debian/kernel/postinst.d/raspi3-firmware). I think this is not used with u-boot or flash-kernel though.
- There's also a Debian live image built with these scripts: https://people.debian.org/~stapelberg/raspberrypi3/2017-03-22/