cmd/buildlet/stage0: move some logging around for the Windows untar mode

Change-Id: I7f5048b6f8e1806cb7801bf903a943eb503b2fc4
Reviewed-on: https://go-review.googlesource.com/51191
Reviewed-by: Jeff Johnson <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: bradfitz

cmd/buildlet/stage0/stage0.go

@@ -54,13 +54,15 @@ func main() {
 		configureSerialLogOutput()
 	}
 	log.SetPrefix("stage0: ")
-	log.Printf("bootstrap binary running.")
 	flag.Parse()
 
 	if *untarFile != "" {
+		log.Printf("running in untar mode, untarring %q to %q", *untarFile, *untarDestDir)
 		untarMode()
+		log.Printf("done untarring; exiting")
 		return
 	}
+	log.Printf("bootstrap binary running")
 
 	switch osArch {
 	case "linux/arm":