x86: coreboot: Add a sample script to build a qemu image

It is useful to boot coreboot (with U-Boot as a payload) from qemu. Add
a sample script to show how to do this.

Signed-off-by: Simon Glass <sjg@chromium.org>

scripts/build-cb.sh

+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0+
+#
+# Sample script to build a disk image suitable for use with coreboot. The image
+# includes a kernel and initrd.
+#
+# YOU WILL NEED to modify this for your needs, e.g. select a kernel.
+#
+# Run this with:
+# qemu-system-i386 -bios coreboot.rom -drive file=disk.img,if=virtio
+
+qemu-img create -f raw disk.img 120M
+mkfs.ext2 -F disk.img
+sudo mkdir -p /mnt/rootfs
+sudo mount -o loop disk.img /mnt/rootfs
+sudo mkdir -p /mnt/rootfs/boot
+sudo cp /boot/vmlinuz /mnt/rootfs/boot/.
+sudo cp /boot/initrd.img /mnt/rootfs/boot/.
+sudo umount /mnt/rootfs