lineage: Rework DTBO image generation

Currently we handle the DTBO images from start to finish, including
AVB footers, and just drop them in $(PRODUCT_OUT)/dtbo.img when done.
While this works for the most part, we are limited in what we can do
with the DTBO images. For example non-A/B device recovery images must
have a copy of the DTBO image appended to recovery (see AOSP docs:
https://source.android.com/devices/bootloader/recovery-image ). This
means that we need to pass the pre-AVB dtbo image into
BOARD_PREBUILT_DTBOIMAGE in order to let the AOSP build system handle it.

This commit reworks the way the DTBO images are built, placing them at
$(PRODUCT_OUT)/dtbo-pre.img instead and setting BOARD_PREBUILT_DTBOIMAGE
to that path so the AOSP build system can pick the images up and sign
them and append to recovery (for applicable devices).

Change-Id: I8819d7cd92ba298de67dbc7f6b321917ba8524f1