ButtonThread: Remove private functions from include

src: Don't pass FileSystem to doErase and doUSBMSD ...

They don't need the filesystem now, so get rid of this arg.

Filesystem: Refer to mbed::FileSystem not LittleFileSystem2 ...

Functions don't need to know about what kind of filesystem we pass it,
so just tell them it's a mbed::FileSystem.

MyUSBMSD: Include Filesystem.h

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

include/MainFilesystem.h: fix header guard ...

Just noticed this header used MAINFS_H, and standard for c++ is
to use the full name of the file, replacing any symbols with _. Let's fix this.

Merge branch 'main' into jookia/cleanup

include: Fix header guard

Code format

Initial commit