Use SPDX headers for SBOM

[rzr]

Would it help to add SPDX headers in all files, including copyrights to comply https://reuse.software ?

I am asking this because i have seen duplicated files in some projects (I know it is bad) and doing some SBOM check, so until code is deduplicated, I am suggesting to consider this in the upstream to prevent this kind of situation when upstream code is vendored

Any opinion? If interested I can detail more

Relate-to: #722