HC preview & GB 2.3.1 use platform-tools rev 2.

Change-Id: Ib2dc3165f88b4c3bd3ba31a956ddb983ea496f2b
diff --git a/sdk/plat_tools_source.properties b/sdk/plat_tools_source.properties
index f23e52a..9528e79 100644
--- a/sdk/plat_tools_source.properties
+++ b/sdk/plat_tools_source.properties
@@ -1,2 +1,2 @@
 Pkg.UserSrc=false
-Pkg.Revision=3
+Pkg.Revision=2