Add License/Copyright to all files

[wnienhaus]

Currently we have only a general LICENSE file in the repo root.

Having license/copyright information in each file makes it very clear for anyone who copies the file what the license/copyright is.

We can either add a short header using the SPDX-License-Identifier format. Or a long one like in the micropython repo.

[wnienhaus]

@ThomasWaldmann commented in the PR (#89):

I'ld rather dislike having the same (long) license text as a header in each file.
If there is a header in a file, I'ld rather expect it to roughly tell what's inside that file and after that there could be also a short notice identifying the license.

So perhaps we'll go the SPDX-License-Identifier route. Final decision is still open.

[dpgeorge]

Here's an example of a short header with description, copyright and license: https://github.com/micropython/micropython/blob/master/lib/littlefs/lfs2.c#L1-L7

[wnienhaus]

That looks pretty neat. One liner description (or longer if needed), followed by (in our case) 1 copyright line, followed by SPDX-License-Identifier: MIT. What do you think @ThomasWaldmann ?

e.g.

# ESP32 ULP Co-Processor Assembler
#
# Copyright 2018-2023, the micropython-esp32-ulp authors, see AUTHORS file.
# SPDX-License-Identifier: MIT

or maybe:

"""
ESP32 ULP Co-Processor Assembler

Copyright 2018-2023, the micropython-esp32-ulp authors, see AUTHORS file.
SPDX-License-Identifier: MIT
"""

even though I wonder whether the 2nd example would be interpreted as a string and use thus more memory?

[ThomasWaldmann]

If one puts the "current year" there, one has to run around once a year and update the years everywhere.

[dpgeorge]

You only need to update the year if you change the code (in that year). It's OK for a copyright to be "old" if the code is also "old".

[wnienhaus]

Btw, saw in some other code today, there's also a SPDX-FileCopyrightText identifier, so we could make it:

# ESP32 ULP Co-Processor Assembler
#
# SPDX-FileCopyrightText: 2018-2023, the micropython-esp32-ulp authors, see AUTHORS file.
# SPDX-License-Identifier: MIT

[wnienhaus]

I have now created PR #95, adding licence headers everywhere.
@ThomasWaldmann @dpgeorge feel free to review and give feedback.