init: Migrate /var/empty to /mnt/empty.

This helps avoid races during chromeos_startup and eliminates the need
for /var/empty to be set immutable.

BUG=chromium:933582
TEST=PreCQ passes

Change-Id: Iecc412fad58f77b7c3f300c2b4b34065111dd498
Reviewed-on: https://chromium-review.googlesource.com/1508836
Commit-Ready: Allen Webb <allenwebb@google.com>
Tested-by: Allen Webb <allenwebb@google.com>
Reviewed-by: Christopher Morin <cmtm@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/init/atrusd.conf b/init/atrusd.conf
index 39d927f..c34eff5 100644
--- a/init/atrusd.conf
+++ b/init/atrusd.conf
@@ -29,7 +29,7 @@
 #
 # -e isn't used because atrusd won't receive libudev events when it's used
 exec minijail0 -u atrus -g atrus -c 2 -i -l -p -n -w -t \
-     -P /var/empty -b / -b /dev -b /sys \
+     -P /mnt/empty -b / -b /dev -b /sys \
      -k '/run,/run,tmpfs,MS_NODEV|MS_NOEXEC|MS_NOSUID,mode=755,size=10M' \
      -b /run/udev -b /run/dbus -b /run/systemd/journal \
      -- /usr/sbin/atrusd \