Rename util/BUILD to as not to confuse Bazel.

Bazel sees BUILD files are markers for different packages. So by having
a file named “BUILD” in the source tree, Bazel thinks that there are
several packages involved.

This change renames it to BUILD.toplevel to avoid this.

Change-Id: Ia76167334cd52f72ff25ecb08533c30e5e423ab8
diff --git a/util/BUILD b/util/BUILD.toplevel
similarity index 100%
rename from util/BUILD
rename to util/BUILD.toplevel