Fix filepath for the rootfs sha256sum

Change-Id: I0b320e089c6062b61baa13b431ace5ddb17dc090
diff --git a/rootfs.mk b/rootfs.mk
index 26a521e..6b7cd9f 100644
--- a/rootfs.mk
+++ b/rootfs.mk
@@ -83,7 +83,7 @@
 	sudo rmdir $(ROOTFS_DIR)
 	sudo sync $(ROOTFS_RAW_IMG)
 	sudo chown ${USER} $(ROOTFS_RAW_IMG)
-	sha256sum $(ROOTFS_RAW_IMG) > $(ROOTFS_RAW_IMG)/rootfs.raw.img.sha256sum
+	sha256sum $(ROOTFS_RAW_IMG) > $(ROOTFS_RAW_IMG).sha256sum
 
 $(ROOTFS_PATCHED_IMG): $(ROOTFS_RAW_IMG)
 	cp -r $(ROOTFS_RAW_IMG) $(ROOTFS_PATCHED_IMG)