optee-rust: set OPTEE_DIR before initializing environment

In the new version of Teaclave TrustZone SDK, OPTEE_DIR is checked
in setup.sh. If OPTEE_DIR is not set, it will download OP-TEE source
code.
Set OPTEE_DIR as the directory of OP-TEE repo.

Signed-off-by: Yuan Zhuang <zhuangyuan04@baidu.com>
Acked-by: Joakim Bech <joakim.bech@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/common.mk b/common.mk
index abf80bd..e19560a 100644
--- a/common.mk
+++ b/common.mk
@@ -536,7 +536,7 @@
 .PHONY: optee-rust
 optee-rust:
 ifeq ($(OPTEE_RUST_ENABLE),y)
-	@(cd $(OPTEE_RUST_PATH) && ./setup.sh)
+	@(export OPTEE_DIR=$(ROOT) && cd $(OPTEE_RUST_PATH) && ./setup.sh)
 endif
 
 ################################################################################