build: tasks: kernel: Add support for MediaTek dtbo images

MediaTek uses "odmdtbo.img" on recent Helio SoCs.

Some MTK targets have a dtbo partition, but not vbmeta partition.
For these, the avb hash footer signing must be omitted.

There are 3 new variables introduced as part of this change:

  TARGET_DTBO_IMAGE_NAME

    The name of your dtbo image, mostly relevant for AVB signing.
    This variable must be set in all cases.

  TARGET_DTBO_IMAGE_TARGET

    The build target (PHONY) that your kernel makefile (Android[Kernel].mk)
    uses to build the dtbo image.

  TARGET_DTBO_IMAGE_PATH

    The output path to the generated dtbo image as dictated by the kernel.
    This shall be relative to $(PRODUCT_OUT).

Change-Id: I52de8cfc38c89d053daeb9f10555479d70f8ac03