Bump copyright year

src: Import and run lang_main ...

This is the bare minimum needed to run this on real hardware.

CMakeLists: Apply compile options to Mbed too ...

This was my original intention but I guess I had things in the wrong order.

sntp: Add SNTP fork ...

This fork of SNTP uses code from https://github.com/howerj/sntp with
many changes by Casey and I to run on mbed.

Added networking support ...

Networking is handled by the NetworkHandler portion of the code, which
provides a few functions for use by other parts of the program, and a
status callback to monitor for connectivity changes. Connectivity is
also handled in an asynchronous mode, to avoid freezing the entire OS
while actions are performed.

* network_connect should be called by any part of the program wishing to
establish connectivity
* network_disconnect should be called to disconnect the network once
done
* network_init is a special function which should only be called by
main. It is responsible for setting the network interface in
asynchronous mode as well as attaching the status monitor callback. No
other part of the program should ever call this.

Lwip is used for the network stack, and IPv6 support has been enabled,
and prefered over IPv4 if available.

CMakeLists: Add option to enable tracing (on by default)

Make use of the mbed tracing subsystem instead of basic printing ...

This enables use of the mbed tracing subsystem for the entire source
code of the project. All the uses of printfs are now appropriately
tagged and assigned a trace level, as in the following example:

[INFO][FS]: Unmounting the filesystem...
[INFO][FS]: OK
[INFO][USBM]: Switching to the usb mass storage mode...
[INFO][USBM]: Disconnecting the usb mass storage device...
[INFO][USBM]: Switched usb mass storage mode off.
[INFO][FS]: Mounting the filesystem...
[INFO][FS]: OK

This was also done in a way that the tracing is thread safe, using a
mutex to lock and unlock as needed.

Note that this cannot be turned on or off yet during build. It is
currently always enabled.

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

Initial commit