bootloader: fdatasync() bootloader configuration

Let's be a bit more conservative here and actually fdatasync() the
configurations we're generating.

I'm seeing an issue at the moment where syslinux isn't finding the
config sometimes, and while I don't think this is the issue, let's try
it.

src/libotutil/ot-gio-utils.h

@@ -69,6 +69,11 @@ gboolean ot_gfile_load_contents_utf8_allow_noent (GFile          *path,
                                                   GCancellable   *cancellable,
                                                   GError        **error);
 
+gboolean ot_gfile_replace_contents_fsync (GFile          *path,
+                                          GBytes         *contents,
+                                          GCancellable   *cancellable,
+                                          GError        **error);
+
 gboolean ot_gfile_ensure_unlinked (GFile         *path,
                                    GCancellable  *cancellable,
                                    GError       **error);