Get build.prop for init_boot.img from the ramdisk props

boot.img and init_boot.img get their build properties from the platform
ramdisk properties.

Test: "m dist" then drop the OTA zip file into
android.github.io/analyseOTA to verify init_boot is present.
Bug: 211823573

Change-Id: I411e62bf0fc3cb49d4b12dadb52d9820fca041fa