volatile-binds: use overlayfs if available
Copying files from the read-only root filesystem to the tmpfs
providing the volatile directories can be slow and waste memory.
If the kernel supports the overlay filesystem, use it to mount
a writable tmpfs on top of the read-only directory from the
rootfs and avoid copies.
Analogous to the modification made to initscripts's
read-only-rootfs-hook in
370fda1b2e8d5dc011522131bba4106de26bfb19.
Signed-off-by: Matt Hoosier <matt.hoosier@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
