x86: coreboot: Enable standard boot

Enable bootstd options and provide instructions on how to boot a linux
distro using coreboot.

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

doc/board/coreboot/coreboot.rst

@@ -67,9 +67,21 @@ To use 4GB of memory, typically necessary for booting Linux distros, add
 In addition to the 32-bit 'coreboot' build there is a 'coreboot64' build. This
 produces an image which can be booted from coreboot (32-bit). Internally it
 works by using a 32-bit SPL binary to switch to 64-bit for running U-Boot. It
-can be useful for running UEFI applications, for example.
+can be useful for running UEFI applications, for example with the coreboot
+build in `$CBDIR`::
+
+   DISK=ubuntu-23.04-desktop-amd64.iso
+   CBDIR=~/coreboot/build
+
+   cp $CBDIR/coreboot.rom.in coreboot.rom
+   cbfstool coreboot.rom add-flat-binary -f u-boot-x86-with-spl.bin \
+      -n fallback/payload -c lzma -l 0x1110000 -e 0x1110000
+
+   qemu-system-x86_64 -m 2G -smp 4 -bios coreboot.rom \
+      -drive id=disk,file=$DISK,if=none \
+      -device ahci,id=ahci \
+      -device ide-hd,drive=disk,bus=ahci.0 \
 
-This has only been lightly tested.
 
 CBFS access
 -----------