boards: arm: nrf9160dk: add deprecation warning for old board names

Add a deprecation warning for nrf9160 DK when the old names
(nrf9160_pca10090, nrf52840_pca10090) are used instead of the
new board name. For nRF9160 SoC: do this for both secure and
non-secure board targets.

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