Default enable CFG_NOTIF_TEST_WD only when dependencies are enabled

Set CFG_NOTIF_TEST_WD ?= y only when the features it needs are enabled.
Fixes the following warning on platforms that enable
CFG_ENABLE_EMBEDDED_TESTS but not CFG_CORE_ASYNC_NOTIF or CFG_CALLOUT:

 mk/config.mk:1039: Warning: Disabling CFG_NOTIF_TEST_WD [requires CFG_CALLOUT CFG_CORE_ASYNC_NOTIF]

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>

mk/config.mk

@@ -1035,7 +1035,9 @@ endif
 CFG_CALLOUT ?= $(CFG_CORE_ASYNC_NOTIF)
 
 # Enable notification based test watchdog
-CFG_NOTIF_TEST_WD ?= $(CFG_ENABLE_EMBEDDED_TESTS)
+ifeq (yyy,$(CFG_ENABLE_EMBEDDED_TESTS)$(CFG_CALLOUT)$(CFG_CORE_ASYNC_NOTIF))
+CFG_NOTIF_TEST_WD ?= y
+endif
 $(eval $(call cfg-depends-all,CFG_NOTIF_TEST_WD,CFG_CALLOUT \
 	 CFG_CORE_ASYNC_NOTIF))