Replace `$(RIOTBOARD)/$(BOARD)` with `$(BOARDDIR)`, which also works for external boards. This allows external boards to build on top of `$(RIOTBOARD)/common/nrf52`.
Replace `$(RIOTBOARD)/$(BOARD)` with `$(BOARDDIR)`, which also works for external boards. This allows external boards to build on top of `$(RIOTBOARD)/common/nrf52`.