image.mk: Generate cpiogz with root-owned files

Some files (e.g. /etc/dropbear) need to be owned by root. Add cpio
option to ensure that.

Other image types (at least targz and squashfs) already have this.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>

include/image.mk

@@ -277,7 +277,7 @@ endif
 
 ifdef CONFIG_TARGET_ROOTFS_CPIOGZ
   define Image/Build/cpiogz
-	( cd $(TARGET_DIR); find . | cpio -o -H newc | gzip -9n >$(BIN_DIR)/$(IMG_PREFIX)-rootfs.cpio.gz )
+	( cd $(TARGET_DIR); find . | cpio -o -H newc -R root:root | gzip -9n >$(BIN_DIR)/$(IMG_PREFIX)-rootfs.cpio.gz )
   endef
 endif