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>

arch/x86/cpu/start.S

@@ -2,6 +2,18 @@
 /*
  *  U-Boot - x86 Startup Code
  *
+ * This is always the first code to run from the U-Boot source. To spell it out:
+ *
+ * 1. When TPL (Tertiary Program Loader) is enabled, the boot flow is
+ * TPL->SPL->U-Boot and this file is used for TPL. Then start_from_tpl.S is used
+ * for SPL and start_from_spl.S is used for U-Boot proper.
+ *
+ * 2. When SPL (Secondary Program Loader) is enabled, but not TPL, the boot
+ * flow is SPL->U-Boot and this file is used for SPL. Then start_from_spl.S is
+ * used for U-Boot proper.
+ *
+ * 3. When neither TPL nor SPL is used, this file is used for U-Boot proper.
+ *
  * (C) Copyright 2008-2011
  * Graeme Russ, <graeme.russ@gmail.com>
  *

arch/x86/cpu/start_from_spl.S

 /* SPDX-License-Identifier: GPL-2.0+ */
 /*
- * 32-bit x86 Startup Code when running from SPL
- *
+ * 32-bit x86 Startup Code when running from SPL. This is the startup code in
+ * U-Boot proper, when SPL is used.
+
  * Copyright 2018 Google, Inc
  * Written by Simon Glass <sjg@chromium.org>
  */

arch/x86/cpu/start_from_tpl.S

 /* SPDX-License-Identifier: GPL-2.0+ */
 /*
- * 32-bit x86 Startup Code when running from TPL
+ * 32-bit x86 Startup Code when running from TPL. This is the startup code in
+ * SPL, when TPL is used.
  *
  * Copyright 2018 Google, Inc
  * Written by Simon Glass <sjg@chromium.org>