init.rc: 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).
unpackbootimg: remove host LOCAL_MODULE_TAG

Change-Id: I199d680dc5ab8bf50f5be65c29095bf3adade695