x86: Rename chromebook-x86 to coreboot

Rename this vendor since it is intended to be used on any platform where
coreboot runs at reset and then loads U-Boot.

So far it is only tested on link. When other boards are supported it is
likely that we will need to move to multiple board names, all under the
'coreboot' vendor. So while it would be possible to remove the vendor for
now, that would be short-sighted.

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