init-live.sh: try to make a union mount when possible

init-live.sh: try to make a union mount when possible

This patch tries to make a union mount in live image. For example,
if aufs is enabled with the aufs-enable.scc configuration fragment,
the init-live.sh script will use aufs to make a union mount.

Although overlayfs is not supported by Yocto kernel yet, this patch
still takes it into consideration with the expect that the related
code should at least serve as a placeholder.

[YOCTO #1487]
[YOCTO #4761]

(From OE-Core rev: 3cd6fedd815688b2f3fd97a56feb5f8696ebeace)

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