volatile-binds: add recipe variable to allow disabling OverlayFS

volatile-binds: add recipe variable to allow disabling OverlayFS

Use the new MOUNT_COPYBIND_AVOID_OVERLAYFS flag provided by mount-copybind.
When SELinux is enabled, processes accessing OverlayFS mounts will get a denial
if the process setting up the mount doesn't have all the permissions that
the accessor has.

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>