Version bump to PD1A.180720.021

Change-Id: If1d4789040a3fc0d64e7cb1a90e715e8524be634
diff --git a/core/build_id.mk b/core/build_id.mk
index 165a441..9e96fc6 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=PD1A.180720.020
+export BUILD_ID=PD1A.180720.021