zephyr: allow to build nRF52840 targets in zephyr-rtos CI

MCUboot was build only for frdm_k64f in zephyr CI.
Extended Zephyr-RTOS integration platform by nrf52840dk_nrf52840
and nrf52840dongle_nrf52840 which allow to build MCUBoot on
these platform in Zephyr-RTOS CI.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>