* nrf: Adding Arduino Primo board support
* nrf: Adding arduino_primo to target boards table in readme.md
* nrf/boards: Activating pyb.LED module for arduino_primo board.
* nrf/boards: Removing define not needed for arduino_primo
Updating arduino_primo board mpconfigboard.h. Removing a define
that was wrongly named. Instead of renaming it, it was removed as
it was never used.
