bootstd: Maintain a list of images

bootstd: Maintain a list of images

We want to keep track of images which are loaded, or those which could
perhaps be loaded. This will make it easier to manage memory allocation,
as well as permit removal of the EFI set_efi_bootdev() feature.

Add a list of these, attached to the bootflow. For now the list is
empty.

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