ui/ncurses: Defer item creation until boot editor is done

ui/ncurses: Defer item creation until boot editor is done

Currently, when creating new menu entries, we create an empty item, add
it to the menu, then run the boot editor. This means that cancelling the
edit will leave an empty item in the menu.

This change defers the creation until the boot editor is done. To do
this, we modify the on_open callback to take a menu rather than an item,
and pass NULL boot data to the editor.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>