cmake: runners dependencies cleanup

This commit renames `FLASH_DEPS` to `RUNNERS_DEPS`.
The current name `FLASH_DEPS` is misleading in the sense that this
depency is added to all runner targets, flash, debug, debugserver,
attach, and not only the flash runners.

Therefore this is now named `RUNNERS_DEPS` instead.

Similar, zephyr_property_target now contains the property
FLASH_DEPENDENCIES, DEBUG_DEPENDENCIES, and so on, so that additional
dependencies can be added for each runner.

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