Some firmware signals the key event without a key being available. The
simple menu then skipped the countdown for that tick, and the basic menu
started waiting for the full timeout again, so firmware that kept
signaling the key event would never boot the default entry.
Read the key inside the wait, and when no key is available keep waiting
on the same timer.
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