When booting up with liveCD image, init scripts can't work well on read-only filesystem. Unionfs,
which is supported in Yocto kernel, allows a filesystem to appear as writeable, but without
actually allowing writes to change the filesystem.
Use unionfs to mount rootfs and make root file system can be writen when using liveCD to boot up.
Set UNION_FS variable depending on kernel config, so that it can work with kernel which doesn't
have unionfs feature.
[RP: Mark recipe as machine specific due to kernel dependency] Signed-off-by: Yang Shi <yang.shi@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
