scripts: kconfig: Make DEPRECATED/EXPERIMENTAL symbols optional

Make `kconfig.py` not crash when any one of these settings is undefined:

  * DEPRECATED
  * EXPERIMENTAL
  * WARN_DEPRECATED
  * WARN_EXPERIMENTAL

While these will continue to exist in Zephyr, they shouldn't be strictly
required for the script to work. One situation in which they could go
missing is when a user creates an application-specific Kconfig root, but
forgets to source "Kconfig.zephyr" inside it - not that this is expected
to work anyway, but it could at least raise a more intelligible error.

Note that when WARN_DEPRECATED is undefined, selecting DEPRECATED will
always produce warnings. Same with WARN_EXPERIMENTAL and EXPERIMENTAL.
This matches the fact that the WARN_* symbols normally have `default y`.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>