releasetools: system_dlkm: add image to dist zip

Bug: 200082547
Test: Manual builds:
      1. Set BOARD_USES_SYSTEM_DLKM_PARTITION
      2. Set BOARD_PREBUILT_SYSTEM_DLKM_IMAGE to external image
      - Check that "m dist" put the system_dlkm image in
      out/dist/aosp_cf_x86_64_phone-img-eng.ramjiyani.zip
Signed-off-by: Ramji Jiyani <ramjiyani@google.com>
Change-Id: I8b7dd40e178db7fe542253bff19040095847fa57