The board name for the nRF52 DK, so far known as nrf52_pca10040, is
renamed to nrf52dk_nrf52832. 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>