Fix rootfs.sh to get artifacts from PRODUCT_OUT

Change-Id: I7deda1810c580460ef6d314c25f1ded68b2822db
diff --git a/kokoro/rootfs.sh b/kokoro/rootfs.sh
index 03be16c..53a6bdc 100644
--- a/kokoro/rootfs.sh
+++ b/kokoro/rootfs.sh
@@ -18,5 +18,5 @@
 
 m docker-rootfs_raw
 
-cp git/out/target/product/imx8m_phanbell/obj/ROOTFS/rootfs.raw.img $KOKORO_ARTIFACTS_DIR
-cp git/out/target/product/imx8m_phanbell/obj/ROOTFS/rootfs.raw.img.sha256sum $KOKORO_ARTIFACTS_DIR
+cp ${PRODUCT_OUT}/obj/ROOTFS/rootfs.raw.img $KOKORO_ARTIFACTS_DIR
+cp ${PRODUCT_OUT}/obj/ROOTFS/rootfs.raw.img.sha256sum $KOKORO_ARTIFACTS_DIR