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
CRs-fixed: 548299

Change-Id: I1b686e2f133970d130e80e81f65687d9533c9eac
Signed-off-by: STELIX <ssspinni@gmail.com>
Build - Fix case where boot/recovery.img were being built with wrong params.

The boot and recovery images  now get built using the same params during
ota package
generation as during a normal build.

Change-Id: I1dd59c659e717a6bd1e4ba21a21e17bd58680963
Signed-off-by: STELIX <ssspinni@gmail.com>