Deprecate USER in favor of BUILD_USERNAME

Bug: 122270019
Test: treehugger, codesearch
diff --git a/core/config.mk b/core/config.mk
index 59b8de0..18f844d 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -90,6 +90,7 @@
   GLOBAL_CFLAGS_NO_OVERRIDE GLOBAL_CPPFLAGS_NO_OVERRIDE \
   ,GCC support has been removed. Use Clang instead)
 $(KATI_obsolete_var DIST_DIR dist_goal,Use dist-for-goals instead. See $(CHANGES_URL)#dist)
+$(KATI_deprecated_var USER,Use BUILD_USERNAME instead. See $(CHANGES_URL)#USER)
 
 # This is marked as obsolete in envsetup.mk after reading the BoardConfig.mk
 $(KATI_deprecate_export It is a global setting. See $(CHANGES_URL)#export_keyword)