Reland "Mark BOARD_BUILD_SYSTEM_ROOT_IMAGE as KATI_obsolete_var"

This reverts commit d6e2c8f84d50e7ae2bf5cd1d6ee3a2ece0a9f97c.

Reason for revert: downstream branches are fixed

Change-Id: I55fa6e8db37d0b7d040679f54be238b512437d29
diff --git a/core/config.mk b/core/config.mk
index e8b984d..22224f6 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -166,6 +166,8 @@
 $(KATI_obsolete_var PRODUCT_SUPPORTS_BOOT_SIGNER,VB 1.0 and related variables are no longer supported)
 $(KATI_obsolete_var PRODUCT_VERITY_SIGNING_KEY,VB 1.0 and related variables are no longer supported)
 $(KATI_obsolete_var BOARD_PREBUILT_PVMFWIMAGE,pvmfw.bin is now built in AOSP and custom versions are no longer supported)
+$(KATI_obsolete_var BOARD_BUILD_SYSTEM_ROOT_IMAGE)
+
 # Used to force goals to build.  Only use for conditionally defined goals.
 .PHONY: FORCE
 FORCE:
@@ -866,11 +868,6 @@
   endif
 endif
 
-# TODO(b/241346584): Mark BOARD_BUILD_SYSTEM_ROOT_IMAGE as KATI_obsolete_var after all users are removed
-ifeq ($(BOARD_BUILD_SYSTEM_ROOT_IMAGE),true)
-    $(error BOARD_BUILD_SYSTEM_ROOT_IMAGE is deprecated)
-endif
-
 ifeq ($(PRODUCT_USE_DYNAMIC_PARTITIONS),true)
     ifneq ($(PRODUCT_USE_DYNAMIC_PARTITION_SIZE),true)
         $(error PRODUCT_USE_DYNAMIC_PARTITION_SIZE must be true for devices with dynamic partitions)