Add variables for configuring Soong plugins

See the Soong commit for detailed usage.

Bug: 76168832
Test: define some variables, use them
Change-Id: Ia8af5b6ec1cb3f12cbc1fee582a309eebb340b7c
diff --git a/core/config.mk b/core/config.mk
index b3f3d36..35a289a 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -178,6 +178,9 @@
 # Set common values
 # ###############################################################
 
+# Initialize SOONG_CONFIG_NAMESPACES so that it isn't recursive.
+SOONG_CONFIG_NAMESPACES :=
+
 # Set the extensions used for various packages
 COMMON_PACKAGE_SUFFIX := .zip
 COMMON_JAVA_PACKAGE_SUFFIX := .jar