bootstd: Drop the bootdev-specific list of bootflows

This list is only used by two functions, which can be updated to iterate
through the global list. Take this approach, which allows the bootdev
list to be dropped.

Overall this makes the code slightly more complicated, but will allow
moving the bootflow list into an alist

Signed-off-by: Simon Glass <sjg@chromium.org>

include/bootdev.h

@@ -109,11 +109,9 @@ struct bootdev_hunter {
  * This is attached to each device in the bootdev uclass and accessible via
  * dev_get_uclass_plat(dev)
  *
- * @bootflows: List of available bootflows for this bootdev
  * @piro: Priority of this bootdev
  */
 struct bootdev_uc_plat {
-	struct list_head bootflow_head;
 	enum bootdev_prio_t prio;
 };