Adds bitbake variable to set additional mount flags for the /var/lib
overlayfs or bind mount when using a read-only root filesystem. This
can be used to set additional options like "-o nodev".
(From OE-Core rev: c3109e40e2c2c881996dd3fcc95fca74f098646d)
Signed-off-by: Colin McAllister <colin.mcallister@garmin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>