Bump build id to 21.12.0

Change-Id: I3be705439d7aab6da6008b38e385fe66502d8c61
diff --git a/core/build_id.mk b/core/build_id.mk
index aebee09..b2a322a 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
+export BUILD_ID=21.12.0