cmake: Allow projects to add additional dependencies to flash target

Using zephyr_target_property::FLASH_DEPENDENCIES to fetch additional
dependencies to the flash operation.

The properties are fetched using a generator expression which allows
users of Zephyr to add dependencies both before and after the flash
target has been defined.

Dependencies can be other targets that must be build / custom commands
which must be executed before the flash operation. Or it can be targets
which must be built.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
Signed-off-by: Haakon Oeye Amundsen <haakon.amundsen@nordicsemi.no>