init: don't double mount /proc and /sys

The first stage init mounts /proc and /sys, and then the second
stage init also mounts /proc and /sys on top of the existing mount.
Only mount these two directories once, in the first stage init.

Not yet fixed: the double mounting of /dev. Removing the double
mounting doesn't work right now because both init stages are trying
to create a property space, and if the double mount of /dev goes away,
the property service in the second stage init fails to work.

Change-Id: I13719027a47526d074390c2b1a605ad99fb43a8f