abuf: Add a helper for initing and allocating a buffer

This construct appears in various places. Reduce code size by adding a
function for it.

It inits the abuf, then allocates it to the requested size.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/scene_textline.c b/boot/scene_textline.c
index 6adef7c..90642a3 100644
--- a/boot/scene_textline.c
+++ b/boot/scene_textline.c
@@ -31,8 +31,7 @@
 			    (struct scene_obj **)&tline);
 	if (ret < 0)
 		return log_msg_ret("obj", -ENOMEM);
-	abuf_init(&tline->buf);
-	if (!abuf_realloc(&tline->buf, max_chars + 1))
+	if (!abuf_init_size(&tline->buf, max_chars + 1))
 		return log_msg_ret("buf", -ENOMEM);
 	buf = abuf_data(&tline->buf);
 	*buf = '\0';