fix timestamps in images to the build date

Pass the -T option to mkuserimg.sh to set all timestamps in the system
image (and any other ext4 image we build) to the value of
ro.build.date.utc for the build.  This makes images produced from a
given target_files bit-identical.

Change-Id: Ibba5fa7a610f476209ef61708729cfd79dece0b6