Move build -> gn/standalone

Make it explicit that those gn files are only for the standalone
build.

Bug: 68710794
Change-Id: I7e2a15219d63b62bf8f249becfdbf39fda2cc183
diff --git a/.gn b/.gn
index 10d8679..d96dec4 100644
--- a/.gn
+++ b/.gn
@@ -1 +1 @@
-buildconfig = "//build/BUILDCONFIG.gn"
+buildconfig = "//gn/standalone/BUILDCONFIG.gn"