init-live.sh: make $ROOT_MOUNT/media writable when necessary
If the live image is mounted as read-only, we cannot make necessary
directories under $ROOT_MOUNT/media, so trying to move the mount points
lead to errors.
So in case that no unification filesystem mechanism is available in kernel
and the rootfs is mounted as read-only, we mount tmpfs on $ROOT_MOUNT/media
so that it's possible to make necessary directories under it.
