x86: Add more comments to the start-up code

The full start-up sequence (TPL->SPL->U-Boot) can be a bit confusing since
each phase has its own 'start' file. Add comments to explain this.

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