scripts/COPY: don't use check_call

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.

scripts/COPY

@@ -16,7 +16,7 @@ def main():
 
     command = ["cp"] + sys.argv[1:-1] + [chroot_target]
 
-    subprocess.check_call(command)
+    sys.exit(subprocess.call(command))
 
 
 if __name__ == "__main__":