pkg: make license declarations spdx compatible

[DanielLockau-MLPA]

Contribution description

• modify license declarations in packages' Makefiles to an SPDX identifier compatible format
• add scripts to validate the packages' license declarations

Testing procedure

The last commit contains the verification scripts. Using the check_packages.sh script on master will fail. Using the same script with all the commits from this PR will succeed.

Issues/PRs references

This PR is related to #21515.

[krzysztof-cabaj]

Thanks for help with SPDX licenses.

I think in Issues/PRs references its better to mentione PR #21515 which tracks SPDX related changes.

[riot-ci]

Murdock results

:x: FAILED

aa3442950cbb63d9b2b7831f26c8b256974ae3c1 dist/tools/licenses: add pkg declaration checker and spdx verifier

Success	Failures	Total	Runtime
3803	0	9919	04m:48s

Artifacts