x86: coreboot: Update doc for CBFS access

Add an example to show how cbfs is used.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[Removed CONFIG_CMD_CBFS from defconfig files]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>