xtest: fix clean default configuration

Default clean configuration missed the output path information
to properly cleanup build artifacts.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>

common.mk

@@ -241,7 +241,8 @@ XTEST_COMMON_FLAGS ?= CROSS_COMPILE_HOST=$(CROSS_COMPILE_NS_USER)\
 xtest-common: optee-os optee-client
 	$(MAKE) -C $(OPTEE_TEST_PATH) $(XTEST_COMMON_FLAGS)
 
-XTEST_CLEAN_COMMON_FLAGS ?= TA_DEV_KIT_DIR=$(OPTEE_OS_TA_DEV_KIT_DIR)
+XTEST_CLEAN_COMMON_FLAGS ?= O=$(OPTEE_TEST_OUT_PATH) \
+	TA_DEV_KIT_DIR=$(OPTEE_OS_TA_DEV_KIT_DIR) \
 
 xtest-clean-common:
 	$(MAKE) -C $(OPTEE_TEST_PATH) $(XTEST_CLEAN_COMMON_FLAGS) clean