files: overlayfs-etc: add overlay mount options to preinit

files: overlayfs-etc: add overlay mount options to preinit

Overlayfs-etc is useful if the rootfs is read-only. One reason to have
the rootfs read-only is to allow image based updating.

Image based updating will change the underlying root file system, which
is unsupported by overlayfs when with some mount options [1].

This disables those options.

[1] https://www.kernel.org/doc/html/latest/filesystems/overlayfs.html?highlight=overlayfs#changes-to-underlying-filesystems

Signed-off-by: Claudius Heine <ch@denx.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>