Enable AB_OTA_UPDATER for mainline

The file $(TARGET_OUT_ETC)/update_engine/update-payload-key.pub.pem
is installed conditionally based on this variable, and was found
to differ between mainline_system and the intended device targets.

Also move the variable from gsi_common.mk as GSI inherits the mainline
boardconfig. The variable is not a PRODUCT flag (and not inherited as
such), so fits better in the BoardConfig I think.

Bug: 80410283
Test: presubmit
Change-Id: Ida6fa923fd01b062ff3b93e7737e1e6edf7e4fa1
diff --git a/target/board/BoardConfigMainlineCommon.mk b/target/board/BoardConfigMainlineCommon.mk
index e13317f..5d67ff5 100644
--- a/target/board/BoardConfigMainlineCommon.mk
+++ b/target/board/BoardConfigMainlineCommon.mk
@@ -39,3 +39,7 @@
 BOARD_AVB_ROLLBACK_INDEX := $(PLATFORM_SECURITY_PATCH_TIMESTAMP)
 
 BOARD_CHARGER_ENABLE_SUSPEND := true
+
+# Enable A/B update
+AB_OTA_UPDATER := true
+AB_OTA_PARTITIONS := system
diff --git a/target/product/gsi_common.mk b/target/product/gsi_common.mk
index fb0478d..2cd57a8 100644
--- a/target/product/gsi_common.mk
+++ b/target/product/gsi_common.mk
@@ -64,10 +64,6 @@
 # Enable dynamic partition size
 PRODUCT_USE_DYNAMIC_PARTITION_SIZE := true
 
-# Enable A/B update
-AB_OTA_UPDATER := true
-AB_OTA_PARTITIONS := system
-
 # Needed by Pi newly launched device to pass VtsTrebleSysProp on GSI
 PRODUCT_COMPATIBLE_PROPERTY_OVERRIDE := true