boards: arm: nrf51_pca10028: rename board to nrf51dk_nrf51422

We rename the nRF51 Dev Kit board target (nrf51_pca10028)
to nrf51dk_nrf51422. We update all associated references
in the supportive documentation and all nRF51-related
cofigurations and overlay files in the samples and tests
in the tree.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>