src/main: reworked the main loop ...

With the latest code cleanup, issues were introduced regarding the mounting and
unmounting of the filesystem.

* Sometimes the board would wake up from sleep and perform mounting and
unmounting of the filesystem in a more or less short loop before going
back to sleep. I suspect this might be due to the sleep getting
interrupted without any button press, aka every 10 minutes or so.
* This code also created a bug where it would create then mount a new
filesystem on the flash after erasing it, breaking the purpose for that
function in the first place which is to erase the flash and nothing
else.

Main is now calling mountFilesystem and unmountFilesystem where
appropriate:

* Before running doUSBMSD, main unmounts the filesystem. When doUSBMSD
is done, main once again mounts the filesystem.
* Before erasing the flash with doERASE, main unmounts the filesystem,
and never mounts it again.
* As a tidying up step, the filesystem gets unmounted, same as the
button thread terminating.

main: Unmount filesystem on shutdown

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.

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