doc: Add a description for bootmeth_extlinux

Add documentation for the extlinux bootmeth.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>

doc/develop/bootstd/extlinux.rst

@@ -0,0 +1,28 @@
+.. SPDX-License-Identifier: GPL-2.0+:
+
+Extlinux Bootmeth
+=================
+
+`Extlinux <https://uapi-group.org/specifications/specs/boot_loader_specification>`_
+(sometimes called syslinux) allows U-Boot to provide a menu of available
+operating systems from which the user can choose.
+
+U-Boot includes a parser for the `extlinux.conf` file. It consists primarily of
+a list of named operating systems along with the kernel, initial ramdisk and
+other settings. The file is stored in the `extlinux/` subdirectory, possibly
+under the `boot/` subdirectory. This list of prefixes (``{"/", "/boot"}`` by
+default) can be selected with the `filename-prefixes` property in the bootstd
+device.
+
+Note that the PXE bootmeth uses the same file format, but in a network context.
+
+When invoked on a bootdev, this bootmeth searches for the file and creates a
+bootflow if found.
+
+When the bootflow is booted, the bootmeth calls ``pxe_setup_ctx()`` to set up
+the context, then ``pxe_process()`` to process the file. Depending on the
+contents, this may boot an operating system or provide a list of options to
+the user, perhaps with a timeout.
+
+The compatible string "u-boot,extlinux" is used for the driver. It is present
+if `CONFIG_BOOTMETH_EXTLINUX` is enabled.