build: create dt.img task

* Remove dt.img creation from generate_extra_images

Change-Id: I676b588cd9acb671771acda638abefee036f0519

build: Don't load dt.img task if custom mkbootimg is used

Change-Id: I7617554a8dc6f44ea0c6a713d834da4fe558caec

core: Account for prebuilt DT images

Change-Id: I52b49fd3e9fc0cb196372e3249c7e3d8b888cdd1

dt_image: Fix build error

build/core/tasks/dt_image.mk:38: *** missing `endif'.  Stop.

Change-Id: If333d378e091ff9d333729dc8a75323966954194

build: only specify least specific directory to dtbtool

New kernels (3.18) place dtb files into the qcom subdirectory,
but older kernels simply create a qcom subdirectory and place
the dtb files at the same level in the heirarchy.  Rather than
looking for the first directory that exists, dtbtool has been
modified to search all subdirectories for dtb files, and
therefore we have to remove the specific subdirectories.

Change-Id: I82e948c0a20605f051a60fdfaae9679e1fa6555e

core: tasks: Use KERNEL_ARCH in dt_image task

* KERNEL_ARCH equals to TARGET_ARCH or TARGET_KERNEL_ARCH if specified

Change-Id: I906e04fd646467ce70da92cb047f0fa0a6ebdb94

build: Use compatibility DTB directory path for custom DTB tool builds

* dtbToolCM is able to search subdirectories, but we are not guaranteed
  that other dtbTool binaries have the same capability. Re-specify
  multiple paths for non-dtbToolCM builds just in case.

Change-Id: Id69e000af5eba60a07662b178cb95950451efd17

build: Add dt.img and kernel target

Change-Id: I4a266d8bd2b01ec4ec696f10f0cc7e912f25a38a

dt_image: Rename target

Change-Id: I2fbe952b55296b6e1a533d071c769788ab3f14ae
Signed-off-by: Brandon McAnsh <brandon.mcansh@gmail.com>