cmake: west: west flash dependencies in sync with CMake flash target

This commit creates a list of a phony targets for each runner, that is:
`west_flash_depends`, `west_debug_depends`, and so on.
Those targets has identical dependencies as CMake runner target.
flash, debug, debugserver, attach targets.

As example `ninja flash` correctly ensures dependencies are taken into
consideration before calling `west flash`.

Unfortunately, calling `west flash` directly would not re-run the flash
dependencies, cause `west flash` would only build the default CMake
target.

Now, `west flash` calls the phony `west_flash_depends` target, ensuring
all deps are up-to-date before flashing (unless --skip-rebuild is given)

The same is true for the other mentioned runners.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>