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

Change-Id: I6cdee7eb1d61ac7d447528962a4fad1a7bbf138d