bootmenu: Allow to quit it via ESC/CTRL+C

When ESC/CTRL+C is pressed interrupt bootmenu and jump into U-Boot console.
As the last entry in bootmenu is always U-Boot console just choose the last
entry when ESC or CTRL+C is pressed.

ESC key is detected when either no other character appears after '\e'
within 10ms or when non-'[' appears after '\e'.

It is useful when bootmenu is part of boot process and you want to
interrupt boot process by scripts which control U-Boot (serial) console.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>