Change build id to PI

Bug: 113306001
diff --git a/core/build_id.mk b/core/build_id.mk
index faabf57..932e214 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=PPR1.180610.009
+export BUILD_ID=PI