Read Soong-exported configuration

This is to ensure that Make and Soong agree on configuration variables
like TARGET_GLOBAL_CFLAGS. Only a few variables are actually checked
currently until we make more actually the same.

Bug: 23566674
Change-Id: Ibede43d933ede4d470e182f9490ea6ec4ef52fbc
diff --git a/core/soong.mk b/core/soong.mk
index df581aa..f1d765c 100644
--- a/core/soong.mk
+++ b/core/soong.mk
@@ -3,6 +3,7 @@
 SOONG_BOOTSTRAP := $(SOONG_OUT_DIR)/.soong.bootstrap
 SOONG_BUILD_NINJA := $(SOONG_OUT_DIR)/build.ninja
 SOONG_IN_MAKE := $(SOONG_OUT_DIR)/.soong.in_make
+SOONG_MAKEVARS_MK := $(SOONG_OUT_DIR)/make_vars-$(TARGET_PRODUCT).mk
 SOONG_VARIABLES := $(SOONG_OUT_DIR)/soong.variables
 
 # Only include the Soong-generated Android.mk if we're merging the