Build: add DT image variable to Makefile

Build: add DT image variable to Makefile

DT image variable is currently present in
generate_extra_images.mk.This file is moved to
build/tasks to support persist image generation
during parallel make. As build/tasks is called
at the end of Makefile, DT image variable is not
available for other images generation like boot and
recovery. Adding this variable in Makefile ensures
the variable is defined before usage

Change-Id: I21f675d8ce648dc1cf1f4f3aede33278300e08c9
CRs-fixed: 548299