There are issues with the current unionfs when making a union mount over "/".
Until these are resolved we can't use unionfs for live booting so disable this
temporarily as a workaround.
unionfs is usable in other circumstances.
[YOCTO #2331 workaround]
(From OE-Core rev: 60ee26ae23132b916019d58e20b8c2e1ddd2b471)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>