The board name for the nRF52840 Dongle, so far known as
nrf52840_pca10059, is renamed to nrf52840dongle_nrf52840. Its
documentation and all references to its name in the tree are updated
accordingly. Overlay and configuration files specific to this board are
also renamed, to match the new board name.
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>