x86: Add a way to detect running from coreboot

x86: Add a way to detect running from coreboot

If U-Boot is running from coreboot we need to skip low-level init. Add
an way to detect this and to set the gd flag.

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