Revert "Mark BOARD_BUILD_SYSTEM_ROOT_IMAGE as KATI_obsolete_var"

This reverts commit 2bd45d0fd8bea17b5180c917cf3afa45bc74e858.

Reason for revert: b/255298902

Change-Id: If0b006b0d88dd08ff116abc50d7b9dd4c8ea5806
diff --git a/core/config.mk b/core/config.mk
index 22224f6..e8b984d 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -166,8 +166,6 @@
 $(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:
@@ -868,6 +866,11 @@
   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)