Check if device already claimed

Client can claim only unclaimed device.
Cloud can claim any device.
Claim with kNone is not allowed.

BUG=25766815, 26143922

Change-Id: Id92168b7f7c290509e672a659f09b7d06af37b76
Reviewed-on: https://weave-review.googlesource.com/1960
Reviewed-by: Vitaly Buka <vitalybuka@google.com>
diff --git a/src/config.h b/src/config.h
index 0c7ea5f..f5a8a77 100644
--- a/src/config.h
+++ b/src/config.h
@@ -21,6 +21,7 @@
 class StorageInterface;
 
 enum class RootClientTokenOwner {
+  // Keep order as it's used with order comparison operators.
   kNone,
   kClient,
   kCloud,