Add a new timeout_style environment variable and a corresponding
GRUB_TIMEOUT_STYLE configuration key for grub-mkconfig. This
controls hidden-timeout handling more simply than the previous
arrangements, and pressing any hotkeys associated with menu entries
during the hidden timeout will now boot the corresponding menu entry
immediately.
GRUB_HIDDEN_TIMEOUT=<non-empty> + GRUB_TIMEOUT=<non-zero> now
generates a warning, and if it shows the menu it will do so as if
the second timeout were not present. Other combinations are
translated into reasonable equivalents.
8f236c1419
