Add BUILD_USERNAME and BUILD_HOSTNAME

These replace $USER and `hostname`, which will soon become stable values
due to sandboxing the build on Linux.

Bug: 122270019
Test: check build.prop
Change-Id: I7493e45a2a2defbdba5d82596cb91d68480f0187
diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index 0fd40db..c4f9a52 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -314,7 +314,7 @@
   # If no BUILD_NUMBER is set, create a useful "I am an engineering build
   # from this date/time" value.  Make it start with a non-digit so that
   # anyone trying to parse it as an integer will probably get "0".
-  BUILD_NUMBER := eng.$(shell echo $${USER:0:6}).$(shell $(DATE) +%Y%m%d.%H%M%S)
+  BUILD_NUMBER := eng.$(shell echo $${BUILD_USERNAME:0:6}).$(shell $(DATE) +%Y%m%d.%H%M%S)
   HAS_BUILD_NUMBER := false
 endif
 .KATI_READONLY := BUILD_NUMBER HAS_BUILD_NUMBER