xenial: Use bindmount script from focal to prevent OTA delta corruptions.

Add script from focal to bindmount overlaid stuff that could have been overwritten by a delta upgrade that did not bring in a system image tarball. The problem here is that the order of unpacking is important, and so system image must come after rootfs image. If there is no system image during updating AND a package with an overriden config is updated in the rootfs that will reset that file to the one from the deb package.