patch the /system/build.prop file last

Works around a problem observed on a retail device: incremental update
from build 1 to build 2 partially completed, leaving a mix of files
from the two builds.  (Why it booted into the regular system instead
of recovery to restart update installation is still a mystery.)
build.prop was one of the files updated, so the device reported itself
as having build 2.  The device hobbled along for months in this state,
until build 3 was released and the 2-to-3 incremental package
repeatedly failed (because some of the files it was trying to patch
were build 1).

This change makes updating build.prop the very last thing does by an
incremental update script, so if installation is aborted and the
regular system starts (and works at all), it will continue reporting
itself as build 1 and be sent the 1-to-2 OTA package again.

Change-Id: I1edc1dcef2bd2495b6fd96517c2f4c574b994f27