README: change reference from LinuxBIOS to coreboot The linuxbios.org website does not exist anymore, so point the reader towards coreboot.org instead. Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> Resolves: https://github.com/openbios/openbios/issues/17 Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org> Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>