Fix NDEF dependency name

The dependency name was incorrectly set to `external-repo` ([following the platform.io example](https://docs.platformio.org/en/latest/manifests/library-json/fields/dependencies.html)) which results in the NDEF library being cloned into a folder called `external-repo`.

It makes more sense to call it `NDEF`; not only to make its description clearer but also to prevent collisions.

Author: hansmbakker