Hello @maniacbug !
The projects under https://github.com/nRF24 are derivative from your work.
Since you have release your library under GPLv2, these libraries are forced to use the same license. While I think the GPLv2 is a great license, it lacks interoperability with others free license like GPLv3 or Apache2.0. The Arduino's core library is already under the GPLv2 or later license, so technically is not a problem, except if i miss something.
What's your opinion about allowing later version of the GPL license?
Example of discussion: nRF24/RF24#227