(towards #1995) Update headers to conform to SPDX (part2)

[sergisiso]

No description provided.

[sergisiso]

Maybe one for you @arporter , this is the same change to the headers but this time to Makefile, x90, and script files.

Importantly, I took a different path for md/readmes. In their case I completely deleted the license and authors. This is because as in the others is not legally required (we have git authoring and a root license), but in this case it can be a minor nuisance for people who read the readmes from a place that doesn't hide comments, and for emedding them in other documentations formats (which I want to start doing more to remove duplications). So I went with the option that I believe has less friction, but let me know if you feel differently.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (72527a7) to head (9312e57).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #3566   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          397       397           
  Lines        55775     55775           
=========================================
  Hits         55775     55775           

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