To make images compatible with each other for development a comon set of keys
will be used. The keys are located on k-stufen.
* add script to download and extract keys
* adjustments to coreos-init-build-env script
* adjustments to check_files_exist function
BREAKING CHANGE: bblayers.conf / bblayers.conf.sample of project
using CoreOS should be adapted to use external-layers instead
of layers for each moved layers