read-only-rootfs-hook.sh: check before bind mounting /var/lib

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]

(From OE-Core rev: 86ac10995fd08226f82d87e23fda5d4898c3190f)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>