build: Add support for device tree in boot.img

Add support for optional device tree image (dt.img)
to boot and recovery images.  Some devices use kernel device
tree and this adds the device tree image as a section within
the boot/recovery images.

Change-Id: I91431ef2f4b86485895678916e39a8572be878eb

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

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: I93d46e11a4245288f0e87c87a2e4bf45ac5aff69

Fix the extra dt.img compilation issue.

Add support for optional device tree image (dt.img)
for device that doesnt have TARGET_BOOTIMAGE_USE_EXT2

Change-Id: I6e07b3ca6d049a8ebdad7ea304b4f39e7c846151

releasetools: Store and use the dt image file through target files

Target files packages may be used for signing images separate from the
build process. Store the device tree image file in the target files
package so it can be used during the signing process.

Change-Id: Ie8507121fa9c4ba57ecffeab05bd859ae5f5b788