[Tracking] SPDX License headers in source files of repos

[dongsupark]

So far in most Flatcar repos, each source file has a common form of copyright headers like:

// Copyright 2021 Kinvolk GmbH
//
// Licensed under the Apache License, Version 2.0 (the "License");

We should make the header community-friendly and consistent all over the repo.

• If possible, use a neutral term to cover all other authors, instead of Kinvolk. Ideally "Copyright The [project name] Authors".
  • We should not specify years, for simplifying maintenance in the future.
  • Keep original copyright statements if they exist.
• Use SPDX License Identifier, instead of having License terms per source file.
• Make sure a LICENSE file on the top-level source directory is correct w.r.t. the SPDX License.
• Avoid using different copyright headers in different files, unless if that is necessary.

For example, each file has a header like that:

// Copyright The project_name Authors
// SPDX-License-Identifier: Apache-2.0

Repos with Apache 2:

• https://github.com/flatcar-linux/afterburn
• https://github.com/flatcar-linux/container-linux-config-transpiler
• https://github.com/flatcar-linux/coreos-cloudinit
• https://github.com/flatcar-linux/flatcar-build-scripts
• https://github.com/flatcar-linux/flatcar-linux-update-operator
• https://github.com/flatcar-linux/flatcar-packer-qemu
• https://github.com/flatcar-linux/ignition
• https://github.com/flatcar-linux/locksmith
• https://github.com/flatcar-linux/mantle (Use SPDX License identifier for source files mantle#263)
• https://github.com/flatcar-linux/mayday
• https://github.com/flatcar-linux/sdnotify-proxy
• https://github.com/flatcar-linux/torcx
• https://github.com/flatcar-linux/toolbox
• https://github.com/flatcar-linux/update-ssh-keys
• https://github.com/flatcar-linux/updateservicectl

Repos with MIT:

• https://github.com/flatcar-linux/flatcar-release-mirror
• https://github.com/flatcar-linux/flatcar-terraform
• https://github.com/flatcar-linux/fleetlock
• https://github.com/flatcar-linux/flog

Repos with BSD or similar:

• https://github.com/flatcar-linux/bootengine (CoreOS)
• https://github.com/flatcar-linux/efunctions (Marcus Downing)
• https://github.com/flatcar-linux/flatcar-dev-util (Chromiums)
• https://github.com/flatcar-linux/init (CoreOS and ChromiumOS)
• https://github.com/flatcar-linux/scripts (CoreOS and ChromiumOS)
• https://github.com/flatcar-linux/seismograph (ChromiumOS)
• https://github.com/flatcar-linux/shim (Matthew Garrett)
• https://github.com/flatcar-linux/update_engine (ChromiumOS)

Repos with GPL or LGPL:

• https://github.com/flatcar-linux/baselayout
• https://github.com/flatcar-linux/fero
• https://github.com/flatcar-linux/grub
• https://github.com/flatcar-linux/nss-altfiles
• https://github.com/flatcar-linux/sysroot-wrappers