BSP import

Remove press detection code

ButtonThread: Use a more descriptive error message

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

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

main: Handle mounting and unmounting for erase and MSD ...

Having the functions handle the filesystem creates an dependency
on the filesystem layer when the functions don't even use it.

Instead, have main handle mounting and unmounting the filesystems.

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.

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.

---
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