hikey: Fixes compiler error for Debian builds

Related to: https://github.com/OP-TEE/optee_os/issues/832



Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (HiKey Debian)
Suggested-by: Machiry Aravind Kumar <makrvcs@gmail.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>

hikey_debian.mk

@@ -136,7 +136,7 @@ ifeq ($(EDK2_CONSOLE_UART),0)
 endif
 
 define edk2-call
-	GCC49_AARCH64_PREFIX=$(AARCH64_CROSS_COMPILE) \
+	GCC49_AARCH64_PREFIX=$(LEGACY_AARCH64_CROSS_COMPILE) \
 	$(MAKE) -j1 -C $(EDK2_PATH) \
 		-f HisiPkg/HiKeyPkg/Makefile $(EDK2_VARS)
 endef