progress: add new BuildLog option

This commit adds a new `BuildLog` option to the `OSBuildOptions`
that can be used to generate a streamed buildlog (e.g. to a file
or a websocket).

This will be used in `ibcli` with a new `--with-buildlog` option.

bib/pkg/progress/syncwriter.go

@@ -0,0 +1,22 @@
+package progress
+
+import (
+	"io"
+	"sync"
+)
+
+type syncedWriter struct {
+	mu *sync.Mutex
+	w  io.Writer
+}
+
+func newSyncedWriter(mu *sync.Mutex, w io.Writer) io.Writer {
+	return &syncedWriter{mu: mu, w: w}
+}
+
+func (sw *syncedWriter) Write(p []byte) (n int, err error) {
+	sw.mu.Lock()
+	defer sw.mu.Unlock()
+
+	return sw.w.Write(p)
+}