zephyr: board nrf51_pca10028 renamed to nrf51dk_nrf51422

Name of the board is changing in the zephyr-rtos:
https://github.com/zephyrproject-rtos/zephyr/pull/23447



This patch aligns the board name used which is required for suppression
of build warnings.

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