Version bump to PPB1.180731.001

Change-Id: I92ce6af898982bb56f7b0b01ec7a394226f179b0
diff --git a/core/build_id.mk b/core/build_id.mk
index 932e214..3f411c3 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=PI
+export BUILD_ID=PPB1.180731.001