x86: Correct wording of coreboot source code

Some files are taken or modified from coreboot, but the files are
no-longer part of the coreboot project. Fix the wording in a few places.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>