- disable cgroups portage feature - allow CAP_NET_ADMIN for the use of systemd-nspawn (https://github.com/systemd/systemd/issues/13308)
subprocess.check_call() raises an execption and produces an ugly Python traceback on the console. It's not really needed. The cp command and Makefile already give error output. Instead we call sys.exit() with the exit status of the cp command.
Makes the Makefile easier to write and the output a little nicer. Inspired by a similar tool ;-)