tool: Print b2s errors to stderr

Useful since b2s often is used like this:

`b2s -c -m app.bin > mgmt_app_digest.h`

hw/application_fpga/tools/b2s/b2s.go

@@ -45,7 +45,7 @@ func main() {
 
 	message, err := os.ReadFile(messageFile)
 	if err != nil {
-		fmt.Printf("%v\n", err)
+		fmt.Fprintf(os.Stderr, "%v\n", err)
 		os.Exit(1)
 	}