system_dlkm: Fix flag for mount point guard

Replace /system_dlkm mount guard flga from
BOARD_USES_SYSTEM_DLKM_PARTITION to
BOARD_USES_SYSTEM_DLKMIMAGE. board_config.mk
and other tools are using this format of image
name to auto generate code to prevent duplication.

Bug: 200082547
Test: TH
Signed-off-by: Ramji Jiyani <ramjiyani@google.com>
Change-Id: Ie4dc899509cf42afb553936e6dbf69d5ce234d01