samples: Add sample that demonstrates a custom board definition

Add a sample that demonstrates a custom board definition. This proves
that BOARD_ROOT works and can be a useful reference when creating a
custom board definition.

Instead of spending time making up a board, the nrf52840_pca10056
board has been copied as-is. And the hello world sample has been used
as the basis for the application.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>