base-files: fix ramoverlay function with kernels 3.18+

Even though there are not many users left within the OpenWrt tree it seems
this function broke during the kernel 3.18 transition.

Fix it by providing a workdir as required by overlayfs.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>

SVN-Revision: 47015