The board target for emulation of nRF52810 on nRF5DK, so far
known as nrf52810_pca10040, is renamed to nrf52dk_nrf52810.
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>