Add soong extra config file

Soong extra config file contains Makefile variables that Soong itself
doesn't need to know. It will be read by product_config module to
provide Makefile variables to build tools e.g. build.prop.

Bug: 346214955
Test: see output of product_config module
Change-Id: I045b2722ae85a10d33b9e6b832f4cc2d6789e59a
diff --git a/core/config.mk b/core/config.mk
index 93c4a50..d957356 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -1253,11 +1253,17 @@
 # consistency with those defined in BoardConfig.mk files.
 include $(BUILD_SYSTEM)/android_soong_config_vars.mk
 
+SOONG_VARIABLES := $(SOONG_OUT_DIR)/soong.$(TARGET_PRODUCT).variables
+SOONG_EXTRA_VARIABLES := $(SOONG_OUT_DIR)/soong.$(TARGET_PRODUCT).extra.variables
+
 ifeq ($(CALLED_FROM_SETUP),true)
 include $(BUILD_SYSTEM)/ninja_config.mk
 include $(BUILD_SYSTEM)/soong_config.mk
 endif
 
+SOONG_VARIABLES :=
+SOONG_EXTRA_VARIABLES :=
+
 -include external/ltp/android/ltp_package_list.mk
 DEFAULT_DATA_OUT_MODULES := ltp $(ltp_packages)
 .KATI_READONLY := DEFAULT_DATA_OUT_MODULES