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/soong_config.mk b/core/soong_config.mk
index dd7e4e6..f799ba0 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -1,5 +1,4 @@
 SOONG_MAKEVARS_MK := $(SOONG_OUT_DIR)/make_vars-$(TARGET_PRODUCT).mk
-SOONG_VARIABLES := $(SOONG_OUT_DIR)/soong.$(TARGET_PRODUCT).variables
 SOONG_ANDROID_MK := $(SOONG_OUT_DIR)/Android-$(TARGET_PRODUCT).mk
 
 include $(BUILD_SYSTEM)/art_config.mk
@@ -356,4 +355,6 @@
 	  rm $(SOONG_VARIABLES).tmp; \
 	fi)
 
+include $(BUILD_SYSTEM)/soong_extra_config.mk
+
 endif # CONFIGURE_SOONG