volatile-binds: Do not create workdir if OverlayFS is disabled

volatile-binds: Do not create workdir if OverlayFS is disabled

When the mountpoint parameter is a directory, the mount-copybind will
first try to use OverlayFS. Because of that, it needs to create the
OverlayFS workdir (determined by the overlay_workdir).

But if the environment variable MOUNT_COPYBIND_AVOID_OVERLAYFS is set
to "1", the script uses bind mount. In that case, the overlay_workdir
is useless, leaving the spec parent directory in a dirty state.

This commit changes mount-copybind so that the overlay_workdir is only
created when MOUNT_COPYBIND_AVOID_OVERLAYFS is not set to 1.

Signed-off-by: Ricardo Simoes <ricardo.simoes@pt.bosch.com>
Signed-off-by: Mark Jonas <mark.jonas@de.bosch.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>