boards: Fix nrf9160 NS flash partition layout

Fix secure and non-secure images overlapping because of incompatible
flash layout configurations.

Align the board configurations to match the nRF9160 DK default
partition layout.

This enforces that the SPU alignment requirement is satisfied for
the nrf9160 MCU.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>