AB_OTA_PARTITIONS: move dtbo under vendor_boot conditional

When building system-only OTA update images, dtbo partition should not
be part of such update.

Kernel build produces vmlinux, DTB, DTBO, and vendor kernel modules.
boot.img contains compressed vmlinux.
vendor_boot.img contains vendor kernel modules + DTB (device tree blob).
dtbo.img contains DTBO (device tree blob overlay).

Since DTBO goes together with DTB, DTBO should be added to AB_OTA_PARTITIONS
only when vendor_boot.img is being built.

Bug: 169968221
Change-Id: I508c2362d207ff18bae1dbc4257bddb92a09b4cd
diff --git a/BoardConfig-common.mk b/BoardConfig-common.mk
index 102234a..9be1fd4 100644
--- a/BoardConfig-common.mk
+++ b/BoardConfig-common.mk
@@ -75,7 +75,6 @@
 
 AB_OTA_PARTITIONS += \
     system \
-    dtbo \
     product \
     vbmeta_system \
     system_ext
@@ -85,6 +84,7 @@
 endif
 ifneq ($(PRODUCT_BUILD_VENDOR_BOOT_IMAGE),false)
 AB_OTA_PARTITIONS += vendor_boot
+AB_OTA_PARTITIONS += dtbo
 endif
 ifneq ($(PRODUCT_BUILD_VBMETA_IMAGE),false)
 AB_OTA_PARTITIONS += vbmeta