updated instructions for creating a minimal rootfs and included build…

…root config for sound driver VM, for building alsactl

examples/virtio-snd/board/odroidc4/snd_driver_vm/README.md

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+<!--
+     Copyright 2024, UNSW
+     SPDX-License-Identifier: CC-BY-SA-4.0
+-->
+
+# Odroid-C4 images
+
+## Linux kernel
+
+### Details
+* Image name: `linux`
+* Config name: `linux_config`
+* Git remote: https://github.com/torvalds/linux.git
+* Tag: v5.18 (commit hash: `4b0986a3613c92f4ec1bdc7f60ec66fea135991f`)
+* Toolchain: `aarch64-none-elf`
+    * Version: GNU Toolchain for the A-profile Architecture 10.2-2020.11 (arm-10.16)) 10.2.1 20201103
+
+You can also get the Linux config used after booting by running the following
+command in userspace: `zcat /proc/config.gz`.
+
+### Instructions for reproducing
+```
+git clone --depth 1 --branch v5.18 https://github.com/torvalds/linux.git
+cp linux_config linux/.config
+make -C linux ARCH=arm64 CROSS_COMPILE=aarch64-none-elf- all -j$(nproc)
+```
+
+The path to the image is: `linux/arch/arm64/boot/Image`.
+
+## RootFS image
+
+### Details
+* Image name: `rootfs.cpio.gz`
+
+### Instructions for reproducing
+
+You can find instructions for building a minimal root filesystem here [here](/docs/VIRTIO.md#minimal-userspace).
+
+You will need to create the following directory structure in the root filesystem for the sound driver VM:
+```
+rootfs/
+├── dev/
+├── sys/
+├── etc/
+    ├── group
+├── modules/
+    ├── led-class.ko
+    ├── snd-hda-codec-generic.ko
+    ├── snd-hda-intel.ko
+    ├── snd.ko
+    ├── ledtrig-audio.ko
+    ├── snd-hda-codec-realtek.ko
+    ├── snd-intel-dspcfg.ko
+    ├── soundcore.ko
+    ├── nls_base.ko
+    ├── snd-hda-codec.ko
+    ├── snd-pcm.ko
+    ├── usbcore.ko
+    ├── snd-ctl-led.ko
+    ├── snd-hda-core.ko
+    ├── snd-timer.ko
+    ├── virtio_snd.ko
+├── alsa/
+├── alsactl
+```
+
+`etc/group`: This file contains the group definitions. The group `audio` should be defined in this file.
+```
+audio:x:0:
+```
+
+`alsa/`, `alsactl`: These files are required for the ALSA sound driver. We will be using buildroot to build these dependencies.
+
+### Details
+* Image name: `rootfs.cpio.gz`
+* Config name: `buildroot_config`
+* Version: 2022.08-rc2
+
+### Instructions for reproducing
+
+```
+wget https://buildroot.org/downloads/buildroot-2022.08-rc2.tar.xz
+tar xvf buildroot-2022.08-rc2.tar.xz
+cp buildroot_config buildroot-2022.08-rc2/.config
+make -C buildroot-2022.08-rc2 alsa-utils
+```
+
+This will generate the `alsa/` and `alsactl` files, which can be found in `buildroot-2022.08-rc2/output/target/usr/share/alsa` and `buildroot-2022.08-rc2/output/target/usr/sbin/alsactl`.