releasetools: build_super_image.py should unpack */build.prop

build_super_image.py attempts to retrieve the "ro.product.device"
property, which requires parsing the */build.prop files from the target
files package. This fixes the following error:
"ExternalError: couldn't resolve ro.product.device"

Test: ./otatools/releasetools/build_super_image.py -p ./otatools
aosp_arm64-target_files.zip super.img

Change-Id: I592524a27b1a4e7544644b1a344071cff9718942