boards: arm: nrf9160_pca10090: rename board to nrf9160dk_nrf9160

We rename the nRF91 Dev Kit board target (nrf9160_pca10090)
to nrf9160dk_nrf9160. We update all associated references
in the supportive documentation and all nRF9160-related
cofigurations and overlay files in the samples and tests
in the tree.

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