Documentation/x86/boot: Reserve type_of_loader=13 for barebox

Documentation/x86/boot: Reserve type_of_loader=13 for barebox

barebox[1], a bootloader for mostly ARM and MIPS embedded systems, can
also be built as EFI payload for x86[2] to provide redundant power-fail
safe, watchdog-supervised boot up.

Since its v2015.09.0 release, it has been booting Linux on x86 with
type_of_loader=0xff[3]. Reserve 13, the next free id, so that can be
used instead in the future.

[1]: https://www.barebox.org/
[2]: https://www.barebox.org/doc/latest/boards/efi.html
[3]: https://elixir.bootlin.com/barebox/v2022.09.0/source/common/efi/payload/image.c#L217

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221002125752.3400831-1-a.fatoum@pengutronix.de