vcontainer: add tiny initramfs image infrastructure

Add proper Yocto image recipes for the tiny initramfs used by
vdkr/vpdmn in the switch_root boot flow:

- vcontainer-tiny-initramfs-image.inc: Shared image configuration
- vcontainer-preinit_1.0.bb: Preinit script package (shared)
- vdkr-tiny-initramfs-image.bb: Tiny initramfs for vdkr
- vpdmn-tiny-initramfs-image.bb: Tiny initramfs for vpdmn

The tiny initramfs contains only busybox and a preinit script that:
1. Mounts devtmpfs, proc, sysfs
2. Mounts the squashfs rootfs.img from /dev/vda
3. Creates tmpfs overlay for writes
4. Performs switch_root to the real rootfs

This replaces ad-hoc file extraction with proper image-based builds,
improving reproducibility and maintainability.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

1 files changed, 70 insertions, 0 deletions

diff --git a/recipes-containers/vcontainer/vcontainer-tiny-initramfs-image.inc b/recipes-containers/vcontainer/vcontainer-tiny-initramfs-image.inc
new file mode 100644
index 00000000..1fef7822
--- /dev/null
+++ b/recipes-containers/vcontainer/vcontainer-tiny-initramfs-image.inc
@@ -0,0 +1,70 @@
+# SPDX-FileCopyrightText: Copyright (C) 2025 Bruce Ashfield
+#
+# SPDX-License-Identifier: MIT
+#
+# vcontainer-tiny-initramfs-image.inc
+# ===========================================================================
+# Shared tiny initramfs image for vdkr/vpdmn switch_root boot flow
+# ===========================================================================
+#
+# This produces a minimal cpio.gz initramfs containing only:
+#   - busybox (for mount, switch_root, sh, etc.)
+#   - vcontainer-preinit (the /init script)
+#
+# Boot flow:
+#   QEMU boots kernel + this initramfs
+#   -> /init (vcontainer-preinit) mounts squashfs rootfs.img from /dev/vda
+#   -> switch_root into rootfs.img with overlayfs
+#   -> ${runtime}-init.sh runs with full container tools
+#
+
+SUMMARY = "Tiny initramfs for vcontainer switch_root"
+DESCRIPTION = "Minimal initramfs containing busybox and preinit script for \
+               mounting squashfs rootfs and performing switch_root."
+LICENSE = "MIT"
+LIC_FILES_CHKSUM = "file://${COMMON_LICENSE_DIR}/MIT;md5=0835ade698e0bcf8506ecda2f7b4f302"
+
+inherit core-image
+
+# Minimal packages: just busybox and the preinit script
+# busybox provides: mount, switch_root, sh, mkdir, sleep, cat, ls, echo, reboot
+PACKAGE_INSTALL = "busybox vcontainer-preinit"
+
+# No extra features - keep it tiny
+IMAGE_FEATURES = ""
+
+# Don't include kernel in initramfs
+PACKAGE_EXCLUDE = "kernel-image-*"
+
+# Output as cpio.gz (standard initramfs format)
+IMAGE_FSTYPES = "${INITRAMFS_FSTYPES}"
+
+# Keep the image small
+IMAGE_ROOTFS_SIZE = "8192"
+IMAGE_ROOTFS_EXTRA_SPACE = "0"
+
+# No locales needed
+IMAGE_LINGUAS = ""
+
+# Suppress the suffix for cleaner naming
+IMAGE_NAME_SUFFIX = ""
+
+# Compatible with x86_64 and aarch64 targets
+COMPATIBLE_HOST = "(x86_64|aarch64).*-linux"
+
+# Create required directories that busybox/preinit expect
+ROOTFS_POSTPROCESS_COMMAND += "create_initramfs_dirs;"
+
+create_initramfs_dirs() {
+    # Create mount points used by preinit
+    install -d ${IMAGE_ROOTFS}/proc
+    install -d ${IMAGE_ROOTFS}/sys
+    install -d ${IMAGE_ROOTFS}/dev
+    install -d ${IMAGE_ROOTFS}/mnt/lower
+    install -d ${IMAGE_ROOTFS}/mnt/upper
+    install -d ${IMAGE_ROOTFS}/mnt/work
+    install -d ${IMAGE_ROOTFS}/mnt/root
+
+    # Create console device node (needed before /dev is mounted)
+    mknod -m 622 ${IMAGE_ROOTFS}/dev/console c 5 1 2>/dev/null || true
+}