x86: Use CONFIG_CHROMEOS_VBOOT for verified boot

x86: Use CONFIG_CHROMEOS_VBOOT for verified boot

At present CONFIG_CHROMEOS is used to determine whether verified boot is
in use. The code to implement that is not in U-Boot mainline.

However, it is useful to be able to boot a Chromebook in developer mode
in U-Boot mainline without needing the verified boot code.

To allow this, use CONFIG_CHROMEOS_VBOOT to indicate that verified boot
should be used, and CONFIG_CHROMEOS to indicate that the board supports
Chrome OS. That allows us to define CONFIG_CHROMEOS on coral.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>