ota: Make blocks image building optional

--block is parsed to python script tools/releasetools/ota_from_target_files, which in turns convert matching partitions build outputs to image files that then are parsed to the end zip. This *should* disable building those images and instead output the classical directory hierarchy to the resulting ota zips.

In simpler terms, instead of having updater-script flashing system.img.new to the phone, this should instead make a "/system" directory available in the zip, where all files should be found there, to be flashed instead.

Of course: setting TARGET_USES_BLOCK_DEVICE_IMAGES := true in the device config should restore this behaviour, but I don't see any actual use for it.