Add XFER_INIT() macro.

src/include/gpxe/xfer.h

@@ -183,6 +183,19 @@ static inline void xfer_init ( struct xfer_interface *xfer,
 	xfer->op = op;
 }
 
+/**
+ * Initialise a static data transfer interface
+ *
+ * @v operations		Data transfer interface operations
+ */
+#define XFER_INIT( operations ) {			\
+		.intf = {				\
+			.dest = &null_xfer.intf,	\
+			.refcnt = NULL,			\
+		},					\
+		.op = operations,			\
+	}
+
 /**
  * Get data transfer interface from generic object communication interface
  *