dexpreopt.config should be created even though unbundled image is built

Bug: 188179858
Test: compare dexpreopt_config.zip files from
 1. TARGET_BUILD_UNBUNDLED_IMAGE=true m dexpreopt_config_zip
 2. m dexpreopt_config_zip
(note that m clean should run between steps)

Change-Id: I92623b59e4fcac397c7fed3d7ab1070fe5281027
diff --git a/core/soong_config.mk b/core/soong_config.mk
index ec67560..1d94661 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -37,6 +37,7 @@
 $(call add_json_bool, Allow_missing_dependencies,        $(filter true,$(ALLOW_MISSING_DEPENDENCIES)))
 $(call add_json_bool, Unbundled_build,                   $(TARGET_BUILD_UNBUNDLED))
 $(call add_json_bool, Unbundled_build_apps,              $(TARGET_BUILD_APPS))
+$(call add_json_bool, Unbundled_build_image,             $(TARGET_BUILD_UNBUNDLED_IMAGE))
 $(call add_json_bool, Always_use_prebuilt_sdks,          $(TARGET_BUILD_USE_PREBUILT_SDKS))
 $(call add_json_bool, Skip_boot_jars_check,              $(SKIP_BOOT_JARS_CHECK))