Bump build id to 21.10.0-rel.2

Change-Id: Id06a18ccfc2412e9b45005eb0e38aa41c89e5803
diff --git a/core/build_id.mk b/core/build_id.mk
index 114a77c..1bfba39 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=21.10.0-rel.1
+export BUILD_ID=21.10.0-rel.2