cros: Add information about booting Chrome OS on x86

Recent versions of Chrome OS do not have a kernel in the root disk, to
save space.

With the improvements to the 'zboot' command it is fairly easy to load
the kernel from the raw partition. Add instructions on how to do this.

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