boards: arm: nrf52840_pca10090: rename board to nrf9160dk_nrf52840

We rename the nRF91 Dev Kit board target for the NRF52840
controller (nrf52840_pca10090) to nrf9160dk_nrf52840. 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. We
also remove an un-referenced board image file that had
erroneously been part of the documentation of this platform.

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