builder: Handle write error on Stopwatch buffer

Use the logger interface to write stopwatch output. With this change,
any error that occurs while trying to write the output is handled
internally by the log module.

This patch also fix a bug where the line break was printed to STDOUT
instead of the stopwatch buffer.

Signed-off-by: Rodrigo Chiossi <rodrigo.chiossi@intel.com>

builder/stopwatch.go

@@ -1,8 +1,8 @@
 package builder
 
 import (
-	"fmt"
 	"io"
+	"log"
 	"time"
 )
 
@@ -21,11 +21,12 @@ type stopWatchEntry struct {
 }
 
 func (sw *stopWatch) Start(name string) {
+	logger := log.New(sw.w, "", log.Ldate|log.Ltime)
 	if sw.w != nil {
 		if len(sw.entries) > 0 {
-			fmt.Println()
+			logger.Println(sw.w, "")
 		}
-		fmt.Fprintf(sw.w, "=> %s\n", name)
+		logger.Printf("=> %s\n", name)
 	}
 	sw.entries = append(sw.entries, stopWatchEntry{name: name})
 	sw.t = time.Now()
@@ -46,6 +47,7 @@ func (sw *stopWatch) Stop() {
 }
 
 func (sw *stopWatch) WriteSummary(w io.Writer) {
+	logger := log.New(w, "", log.Ldate|log.Ltime)
 	if len(sw.entries) == 0 {
 		return
 	}
@@ -56,10 +58,10 @@ func (sw *stopWatch) WriteSummary(w io.Writer) {
 		}
 	}
 	var sum time.Duration
-	fmt.Fprintf(w, "\nTIMINGS\n")
+	logger.Printf("\nTIMINGS\n")
 	for _, e := range sw.entries {
-		fmt.Fprintf(w, "  %-*s %s\n", max, e.name, e.d.Truncate(time.Millisecond))
+		logger.Printf("  %-*s %s\n", max, e.name, e.d.Truncate(time.Millisecond))
 		sum += e.d
 	}
-	fmt.Fprintf(w, "TOTAL: %s\n", sum.Truncate(time.Millisecond))
+	logger.Printf("TOTAL: %s\n", sum.Truncate(time.Millisecond))
 }