Create /storage mountpoint so Dalvik can mark as slave in zygotes

Mount a tmpfs as /storage in order to create a mountpoint, so that Dalvik
can later mark /storage as slave in zygotes.  Must use tmpfs as rootfs
can't be bind mounted (otherwise "mount rootfs /storage /storage bind"
would work as an alternative).

Change-Id: I7d3bd56dcebaa54c0938ef9193e5df992e636a92