cmake: flash: scripts: Include externally built hex files

Allow user to add externally built hex files to the cmake property
HEX_FILES_TO_MERGE. The hex files in this list will be merged
with the hex file generated when building the zephyr application.

This allows users to leverage the application configuration
available in Kconfig and CMake to help decide what hex file
should be merged with the zephyr application.

Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>