Add BOARD_BUILD_SYSTEM_ROOT_IMAGE to config vars

It'll be used from init_first_stage.

Bug: 187196593
Test: build
Change-Id: I4c5374debd56fbf9d11b456498835a9c411d01d6
diff --git a/core/android_soong_config_vars.mk b/core/android_soong_config_vars.mk
index 74558f5..fc153bb 100644
--- a/core/android_soong_config_vars.mk
+++ b/core/android_soong_config_vars.mk
@@ -28,6 +28,7 @@
 
 $(call add_soong_config_var,ANDROID,TARGET_ENABLE_MEDIADRM_64)
 $(call add_soong_config_var,ANDROID,BOARD_USES_ODMIMAGE)
+$(call add_soong_config_var,ANDROID,BOARD_BUILD_SYSTEM_ROOT_IMAGE)
 
 # TODO(b/172480615): Remove when platform uses ART Module prebuilts by default.
 ifeq (,$(filter art_module,$(SOONG_CONFIG_NAMESPACES)))