[refcnt] Add ref_init() wrapper function

Standardise on using ref_init() to initialise an embedded reference
count, to match the coding style used by other embedded objects.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/image.c

@@ -64,7 +64,7 @@ struct image * alloc_image ( void ) {
 
 	image = zalloc ( sizeof ( *image ) );
 	if ( image ) {
-		image->refcnt.free = free_image;
+		ref_init ( &image->refcnt, free_image );
 	}
 	return image;
 }