add license headers to files containing code

[jrobeson]

Currently it's not clear what license these files are released under, Arduino's core libraries are licensed under the LGPL 2.1+

[rogerclarkmelbourne]

I cant see any licenses in the Nordic SDK either, and in the SDK's on Nordic's site there is a license.txt file, which I presume should have been included in this repo.

PS. I'm trying to work precisely which version of the SDK that was used. It doesnt look like it was v9 or v8 as the softdevice/ common files don't match with either of those SDK versions