[PATCHv2] doc/develop/distro.rst: Better document upstream definition of extlinux.conf

[Tom Rini]

First, the "Boot Loader Specification" link has moved to a new location,
so link to that directly. Second, that link does not document as much of
the extlinux.conf format as I recall the old version doing at least.
However, the Syslinux Project wiki is the current location of the documentation
linked to in doc/README.pxe and also has a reference for SYSLINUX. Link
to both of these.

Signed-off-by: Tom Rini <trini at konsulko.com>
---
Changes in v2:
- As Simon noted there is a SYSLINUX page on the Syslinux project wiki
  as well that documents other options, reword the introductory
  paragraph more and link to both SYSLINUX and PXELINUX and then our
  deviances from that.
---
 doc/develop/distro.rst | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/doc/develop/distro.rst b/doc/develop/distro.rst
index 637bc27fc2d7..1d2f9c4c32bc 100644
--- a/doc/develop/distro.rst
+++ b/doc/develop/distro.rst
@@ -67,11 +67,16 @@ Boot Configuration Files
 ------------------------
 
 The standard format for boot configuration files is that of extlinux.conf, as
-handled by U-Boot's "syslinux" (disk) or "pxe boot" (network). This is roughly
-as specified at `Boot Loader Specification`_:
+handled by U-Boot's "syslinux" (disk) or "pxe boot" (network). This format is
+not formally standardized and documented in a single location. However, other
+implementations do document it and we attempt to be as compatible as possible.
 
+* The UAPI Group Specifications `Boot Loader Specification`_
 
-... with the exceptions that the Boot Loader Specification document:
+* The Syslinux Project documents both `PXELINUX`_ and `SYSLINUX`_ files and is
+  the originator of the format.
+
+That said, we have some differences to these documents, namely:
 
 * Prescribes a separate configuration per boot menu option, whereas U-Boot
   lumps all options into a single extlinux.conf file. Hence, U-Boot searches
@@ -440,7 +445,9 @@ way in future u-boot versions.  In particular the <device type>_boot
 variables (e.g. mmc_boot, usb_boot) are a strictly internal implementation
 detail and must not be used as a public interface.
 
-.. _`Boot Loader Specification`: https://systemd.io/BOOT_LOADER_SPECIFICATION/
+.. _`Boot Loader Specification`: https://uapi-group.org/specifications/specs/boot_loader_specification/
+.. _`PXELINUX`: https://wiki.syslinux.org/wiki/index.php?title=PXELINUX
+.. _`SYSLINUX`: https://wiki.syslinux.org/wiki/index.php?title=SYSLINUX
 
 .. sectionauthor:: (C) Copyright 2014 Red Hat Inc.
 .. sectionauthor:: Copyright (c) 2014-2015, NVIDIA CORPORATION.  All rights reserved.
-- 
2.43.0