Bump build id to 21.12.0-rel.1

Change-Id: I9fe8e1779520411b0ee5cc1a28c7f20ba8bfe352
diff --git a/core/build_id.mk b/core/build_id.mk
index efa4e1c..20ad686 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=21.12.0-rel.0
+export BUILD_ID=21.12.0-rel.1