Tune ROOTFS_PATCHED_IMG target

If the recipe fails for whatever reason one can not do further
incremental builds reliably. The output, ROOTFS_PATCHED_IMG, is
created at the top of the recipe. Next build it's current and in
a bad state. Instead start with ROOTFS_RAW_IMG and only create
ROOTFS_PATCHED_IMG when all is done.

Change-Id: I7e4cb95d3373cc20caacc15f1b0512112ce4ee76
diff --git a/rootfs.mk b/rootfs.mk
index 6ac3c0e..1e29127 100644
--- a/rootfs.mk
+++ b/rootfs.mk
@@ -129,11 +129,11 @@
                        $(ROOTDIR)/cache/packages.tgz \
                        | $(PRODUCT_OUT)/boot.img \
                          modules
-	cp -r $(ROOTFS_RAW_IMG) $(ROOTFS_PATCHED_IMG)
+	cp $(ROOTFS_RAW_IMG) $(ROOTFS_PATCHED_IMG).wip
 	mkdir -p $(ROOTFS_DIR)
 	-sudo umount $(ROOTFS_DIR)/boot
 	-sudo umount $(ROOTFS_DIR)
-	sudo mount -o loop $(ROOTFS_PATCHED_IMG) $(ROOTFS_DIR)
+	sudo mount -o loop $(ROOTFS_PATCHED_IMG).wip $(ROOTFS_DIR)
 	sudo mount -o loop $(PRODUCT_OUT)/boot.img $(ROOTFS_DIR)/boot
 
 	sudo cp $(ROOTDIR)/board/fstab.emmc $(ROOTFS_DIR)/etc/fstab
@@ -149,8 +149,9 @@
 	sudo umount $(ROOTFS_DIR)/boot
 	sudo umount $(ROOTFS_DIR)
 	sudo rmdir $(ROOTFS_DIR)
-	sudo sync $(ROOTFS_PATCHED_IMG)
-	sudo chown ${USER} $(ROOTFS_PATCHED_IMG)
+	sudo sync $(ROOTFS_PATCHED_IMG).wip
+	sudo chown ${USER} $(ROOTFS_PATCHED_IMG).wip
+	mv $(ROOTFS_PATCHED_IMG).wip $(ROOTFS_PATCHED_IMG)
 
 $(PRODUCT_OUT)/rootfs.img: $(HOST_OUT)/bin/img2simg $(ROOTFS_PATCHED_IMG)
 	$(HOST_OUT)/bin/img2simg $(ROOTFS_PATCHED_IMG) $(PRODUCT_OUT)/rootfs.img