Bump build id to 21.05.0-rel.1

Change-Id: I4e97c4f55c5e74e64275efe8e846a6e18903dd2a
diff --git a/core/build_id.mk b/core/build_id.mk
index f477cb3..74322a4 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=21.05.0-rel.0
+export BUILD_ID=21.05.0-rel.1