zephyr: add slot-1 in RAM confg for nrf52840dk_nrf52840

Added configuration for emulate the secondary image flash.
Thanks to that it is easy to test mcuboot behavior while
the secondary flash device stop working.
Need additionally to modify the zephyr/drivers/flash/flash_simulator.c
driver, soflash_sim_read() always returns an error e.g: -EINVAL.

For building mcuboot use following comand-line:
west build -d build/mcuboot_y -b nrf52840dk_nrf52840 \
bootloader/mcuboot/boot/zephyr/ -- \
-DDTC_OVERLAY_FILE=./boards/nrf52840dk_ram.overlay \
-DOVERLAY_CONFIG=./boards/flash_sim_driver.conf

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