qemu_v8.mk: Add option to use U-Boot instead of edk2

Add an option to build and use U-Boot instead of edk2 as BL33
in the bootflow. To use this option: make UBOOT=y

Signed-off-by: Ruchika Gupta <ruchika.gupta@linaro.org>
Reviewed-by: Jerome Forissier <jerome@forissier.org>
Tested-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/common.mk b/common.mk
index 2d2e43b..5ad5b41 100644
--- a/common.mk
+++ b/common.mk
@@ -29,6 +29,7 @@
 UNAME_M				:= $(shell uname -m)
 BUILD_PATH			?= $(ROOT)/build
 LINUX_PATH			?= $(ROOT)/linux
+UBOOT_PATH			?= $(ROOT)/u-boot
 OPTEE_OS_PATH			?= $(ROOT)/optee_os
 OPTEE_CLIENT_PATH		?= $(ROOT)/optee_client
 OPTEE_TEST_PATH			?= $(ROOT)/optee_test