Add support to build system_dlkm.img

There are 2 choices to build system_dlkm.img for
the system_dlkm partition for Android T launch
devices and must choose one.

1. Use kernel prebuilt system_dlkm.img
   - BOARD_PREBUILT_SYSTEM_DLKM_IMAGE to point image
2. Build from kernel prebuilt system_dlkm_staging
   - PRODUCT_BUILD_SYSTEM_DLKM_IMAGE

Both requires: BOARD_SYSTEM_DLKM_PARTITION_SIZE and
must be 64MB or higher in size (enforced via vts).

Bug: 200082547
Test: TH
Test: atest vts_system_dlkm_partition_test
Signed-off-by: Ramji Jiyani <ramjiyani@google.com>
Change-Id: I83435123bd8aa3d04ab8a8b650a95fbab0bc49f2