Configure APT for stretch

1. Add Machinekit repository

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 43DDF224
sudo sh -c \
  "echo 'deb http://deb.machinekit.io/debian stretch main' > \
  /etc/apt/sources.list.d/machinekit.list"
sudo apt-get update