Bump copyright year

ButtonThread: Use a more descriptive error message

ButtonThread: Remvoe unused flag

Added SPDX license informations to all files. ...

The only files where this isn't present are the LICENSE.md for obvious
reasons and the mbed_app.json, as comments are not allowed in json.

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Changes v1 -> v2:
- fixed capitalisation of the copyright word in the script directory and in the cmake list
- modified the source code and header files so the SPDX info is
in a multi-line comment block

Code format

Initial commit