Makefile: Use common code for DM_ETH deprecation warning

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

Tested with snow

Old message:

===================== WARNING ======================
This board does not use CONFIG_DM_ETH (Driver Model
for Ethernet drivers). Please update the board to use
CONFIG_DM_ETH before the v2020.07 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_DM_ETH (Driver Model
for Ethernet drivers). Please update the board to use
CONFIG_DM_ETH before the v2020.07 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>
diff --git a/Makefile b/Makefile
index 31af66f..0613f6d 100644
--- a/Makefile
+++ b/Makefile
@@ -1100,17 +1100,7 @@
 		$(CONFIG_SPI_FLASH))
 	$(call deprecated,CONFIG_WDT,DM watchdog,v2019.10,\
 		$(CONFIG_WATCHDOG)$(CONFIG_HW_WATCHDOG))
-ifneq ($(CONFIG_NET),)
-ifneq ($(CONFIG_DM_ETH),y)
-	@echo >&2 "===================== WARNING ======================"
-	@echo >&2 "This board does not use CONFIG_DM_ETH (Driver Model"
-	@echo >&2 "for Ethernet drivers). Please update the board to use"
-	@echo >&2 "CONFIG_DM_ETH before the v2020.07 release. Failure to"
-	@echo >&2 "update by the deadline may result in board removal."
-	@echo >&2 "See doc/driver-model/migration.rst for more info."
-	@echo >&2 "===================================================="
-endif
-endif
+	$(call deprecated,CONFIG_DM_ETH,Ethernet drivers,v2020.07,$(CONFIG_NET))
 	@# Check that this build does not use CONFIG options that we do not
 	@# know about unless they are in Kconfig. All the existing CONFIG
 	@# options are whitelisted, so new ones should not be added.