read-only-rootfs-hook.sh: check before bind mounting /var/lib
It's possible that /var/lib is on a separate writable partition. In such
situation, we should not bind mount /var/lib with tmpfs, becasue it's
already writable.
This patch fixes this problem by checking whether /var/lib is already
on a writable partition.
[YOCTO #4888]
Signed-off-by: Chen Qi <Qi.Chen@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
