RootConfiguration derived Default, which set menu-timeout to 0 and the
version to 0, while an empty configuration file gets a 10 second
timeout and the latest version. --autoconfigure uses the default
configuration, so it booted the first entry without showing the menu.
Implement Default with the same values serde uses.
The boot menu is now a BootMenu trait with two styles. The basic menu is
the existing one that prints entries and selects them by number. The new
simple menu is full-screen: entries are centered and highlighted, the
arrow keys, home, end, page up and page down move the selection, enter
boots it, and any key stops the countdown.
The style is picked with --menu-style or menu-style in [options], and
defaults to simple. The timer code used to wait for a key is shared
between both menus in menu.rs.
alex