Makefile: Use common code for WDT deprecation warning

Makefile: Use common code for WDT deprecation warning

Update the CONFIG_WDT check to use the 'deprecated' function.

Tested with kmcent2

Old message:

===================== WARNING ======================
This board does not use CONFIG_WDT (DM watchdog support).
Please update the board to use CONFIG_WDT before the
v2019.10 release.
Failure to update by the deadline may result in board removal.
See doc/driver-model/migration.rst for more info.
====================================================

New message:

===================== WARNING ======================
This board does not use CONFIG_WDT (Driver Model
for DM watchdog). Please update the board to use
CONFIG_WDT before the v2019.10 release. Failure to
update by the deadline may result in board removal.
See doc/driver-model/migration.rst for more info.
====================================================

Signed-off-by: Simon Glass <sjg@chromium.org>