Create custom images for Revolution Pi

Intended usage:

* Download Raspbian image, works with both desktop and lite images:
  curl -O http://downloads.raspberrypi.org/raspbian/images/raspbian-2019-04-09/2019-04-08-raspbian-stretch.zip
  curl -O http://downloads.raspberrypi.org/raspbian_lite/images/raspbian_lite-2019-04-09/2019-04-08-raspbian-stretch-lite.zip
  unzip *-raspbian-*.zip
  rm *-raspbian-*.zip

* Customize for Revolution Pi
  (requires root, an armhf system (RasPi or VM) and Internet connectivity;
  to cross-build, apt-get install qemu-user-static binfmt-support;
  custom packages can be placed in debs-to-install/):
  customize_image.sh <raspbian-image>

* Collect sources on a physical medium for GPL compliance
  (requires root and Internet connectivity):
  collect_sources.sh <raspbian-image> /media/usbstick