Add RBE mk file to support bootstrapping RBE and setting relevant
environment variables.

Test: Built aosp_arm-user with and without USE_RBE. With USE_RBE,
used a simple proxy script in place of rewrapper.

Change-Id: Ifb6bfd9f7292346f4750a6a409aa0def4e3c0363
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 57ab331..eae66c5 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -152,6 +152,7 @@
 $(call add_json_bool, MinimizeJavaDebugInfo,             $(filter true,$(PRODUCT_MINIMIZE_JAVA_DEBUG_INFO)))
 
 $(call add_json_bool, UseGoma,                           $(filter-out false,$(USE_GOMA)))
+$(call add_json_bool, UseRBE,                            $(filter-out false,$(USE_RBE)))
 $(call add_json_bool, Arc,                               $(filter true,$(TARGET_ARC)))
 
 $(call add_json_list, NamespacesToExport,                $(PRODUCT_SOONG_NAMESPACES))