core/Makefile: system_dlkm: Copy image for dist

Add support to copy built system_dlkm to IMAGES;
or prebuilt image to PREBUILD_IMAGES for release
tools to pick up for the 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 "m" builds the system_dlkm.img in both cases;
        and drops image at out/target/product/vsoc_x86_64/
      - Check that "m dist" can find the image in
        $(zip_root)/IMAGES and $(zip_root)/PREBUILT_IMAGES
Test: launch_cvd && atest vts_system_dlkm_partition_test
Signed-off-by: Ramji Jiyani <ramjiyani@google.com>
Change-Id: I77dfcc51746fd97cb205aa9c139a08f5cdf3d9ea