zephyr:boards nrf52840_pca10056 rename to nrf52840dk_nrf52840

Board was renamed here:
https://github.com/zephyrproject-rtos/zephyr/pull/23524/



Patch aligns the name in MCUBoot and decouples one of nrf52840 dedicated
overlay from exact board name.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>