Add /postinstall partition when using the A/B updater.

The new top level directory /postinstall is used by the A/B updater to
mount the new partition and run a post-install program before rebooting
into that new system.

init.rc was extended to label this new directory since the initrd has
no extended attributes.

Bug: 27177071
TEST=`ls -laZ /` shows the /postinstall directory on edison-eng

(cherry picked from commit bb968fb0)

Change-Id: Iff993135c7ce3a1a0f6450892ef7382da408fd5e