Traceability to source

It would be really helpful to be able to trace an arbitrary running version of MicroPython on an arbitrary micro:bit, back to the github source that generated it.

This would be especially useful when diagnosing problems that users have, to say 'tell me the version string that you get (when you type this in) or (when you press the reset button), as anyone could then look through github for that version and see if a particular feature or bug-fix is, or is not, implemented.

Also, it's quite important to be able to setup an environment when reproducing issues, to be using the same version of micro:bit MicroPython as others are using. So, if there is an easy(ish) process by which that identifying information can be followed to a cached released .hex that could just be downloaded and used to reproduce the issue, that would additionally help.

The startup message that you get when you press the reset button seems to identify the root MicroPython version, but not the actual build of the micro:bit version that was based on that code.