Version bump to MOB31Y

Change-Id: I9e9875a602da3c0d7411ea5fe0942f435638e675
diff --git a/core/build_id.mk b/core/build_id.mk
index ba3b67a..d29dcab 100644
--- a/core/build_id.mk
+++ b/core/build_id.mk
@@ -18,4 +18,4 @@
 # (like "CRB01").  It must be a single word, and is
 # capitalized by convention.
 
-export BUILD_ID=MOB31X
+export BUILD_ID=MOB31Y