Fix incorrect path to simple.script

Fix bug reported in https://github.com/OP-TEE/optee_os/issues/542

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
diff --git a/hikey.mk b/hikey.mk
index b02e0fa..f509c3e 100644
--- a/hikey.mk
+++ b/hikey.mk
@@ -251,7 +251,7 @@
 
 	@echo "# Script called by udhcpc (DHCP client) to update the network configuration" >> $(GEN_ROOTFS_FILELIST)
 	@echo "dir /etc/udhcp 755 0 0" >> $(GEN_ROOTFS_FILELIST)
-	@echo "file /etc/udhcp/simple.script $(GEN_ROOTFS_PATH)/busybox/examples/udhcp/simple.script 755 0 0" >> $(GEN_ROOTFS_FILELIST)
+	@echo "file /etc/udhcp/simple.script $(ROOT)/busybox/examples/udhcp/simple.script 755 0 0" >> $(GEN_ROOTFS_FILELIST)
 	@echo "" >> $(GEN_ROOTFS_FILELIST)
 
 	@echo "# Run this manually after plugging a USB to ethernet adapter" >> $(GEN_ROOTFS_FILELIST)