Makefile: support build and avb signing of dtbo img

Test: make with corresponding configurations in BoardConfig.mk
- dtbo.img is copied to the OUT directory
- avbtool info_image --image dtbo.img shows avb hashes
- avbtool info_image --image vbmeta.img shows dtbo partition info

Bug: 37730955
Change-Id: If3edaab860926c743f18fd031c911b076eb09fab