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/Makefile b/core/Makefile
index 179cf29..56b45df 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -317,7 +317,7 @@
 # The string used to uniquely identify the combined build and product; used by the OTA server.
 ifeq (,$(strip $(BUILD_FINGERPRINT)))
   ifeq ($(strip $(HAS_BUILD_NUMBER)),false)
-    BF_BUILD_NUMBER := $(USER)$$($(DATE_FROM_FILE) +%m%d%H%M)
+    BF_BUILD_NUMBER := $(BUILD_USERNAME)$$($(DATE_FROM_FILE) +%m%d%H%M)
   else
     BF_BUILD_NUMBER := $(file <$(BUILD_NUMBER_FILE))
   endif
@@ -420,6 +420,8 @@
 	        BUILD_ID="$(BUILD_ID)" \
 	        BUILD_DISPLAY_ID="$(BUILD_DISPLAY_ID)" \
 	        DATE="$(DATE_FROM_FILE)" \
+	        BUILD_USERNAME="$(BUILD_USERNAME)" \
+	        BUILD_HOSTNAME="$(BUILD_HOSTNAME)" \
 	        BUILD_NUMBER="$(BUILD_NUMBER_FROM_FILE)" \
 	        BOARD_BUILD_SYSTEM_ROOT_IMAGE="$(BOARD_BUILD_SYSTEM_ROOT_IMAGE)" \
 	        AB_OTA_UPDATER="$(AB_OTA_UPDATER)" \