feat: custom kernel build + boot fixes for working container runtime

Build a custom Tiny Core 17.0 kernel (6.18.2) with missing configs
that the stock kernel lacks for container workloads:
- CONFIG_CGROUP_BPF=y (cgroup v2 device control via BPF)
- CONFIG_DEVTMPFS=y (auto-create /dev device nodes)
- CONFIG_DEVTMPFS_MOUNT=y (auto-mount devtmpfs)
- CONFIG_MEMCG=y (memory cgroup controller for memory.max)
- CONFIG_CFS_BANDWIDTH=y (CPU bandwidth throttling for cpu.max)

Also strips unnecessary subsystems (sound, GPU, wireless, Bluetooth,
KVM, etc.) for minimal footprint on a headless K8s edge appliance.

Init system fixes for successful boot-to-running-pods:
- Add switch_root in init.sh to escape initramfs (runc pivot_root)
- Add mountpoint guards in 00-early-mount.sh (skip if already mounted)
- Create essential device nodes after switch_root (kmsg, console, etc.)
- Enable cgroup v2 controller delegation with init process isolation
- Mount BPF filesystem for cgroup v2 device control
- Add mknod fallback from sysfs in 20-persistent-mount.sh for /dev/vda
- Move KubeSolo binary to /usr/bin (avoid /usr/local bind mount hiding)
- Generate /etc/machine-id in 60-hostname.sh (kubelet requires it)
- Pre-initialize iptables tables before kube-proxy starts
- Add nft_reject, nft_fib, xt_nfacct to kernel modules list

Build system changes:
- New build-kernel.sh script for custom kernel compilation
- Dockerfile.builder adds kernel build deps (flex, bison, libelf, etc.)
- Selective kernel module install (only modules.list + transitive deps)
- Install iptables-nft (xtables-nft-multi) + shared libs in rootfs

Tested: ISO boots in QEMU, node reaches Ready in ~35s, CoreDNS and
local-path-provisioner pods start and run successfully.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

init/lib/20-persistent-mount.sh

@@ -8,11 +8,26 @@ if [ "$KUBESOLO_NOPERSIST" = "1" ]; then
     return 0
 fi
 
+# Load block device drivers before waiting (modules loaded later in stage 30,
+# but we need virtio_blk available NOW for /dev/vda detection)
+modprobe virtio_blk 2>/dev/null || true
+# Trigger mdev to create device nodes after loading driver
+mdev -s 2>/dev/null || true
+
+# Fallback: create device node from sysfs if devtmpfs/mdev didn't
+DEV_NAME="${KUBESOLO_DATA_DEV##*/}"
+if [ ! -b "$KUBESOLO_DATA_DEV" ] && [ -f "/sys/class/block/$DEV_NAME/dev" ]; then
+    MAJMIN=$(cat "/sys/class/block/$DEV_NAME/dev")
+    mknod "$KUBESOLO_DATA_DEV" b "${MAJMIN%%:*}" "${MAJMIN##*:}" 2>/dev/null || true
+    log "Created $KUBESOLO_DATA_DEV via mknod ($MAJMIN)"
+fi
+
 # Wait for device to appear (USB, slow disks, virtio)
 log "Waiting for data device: $KUBESOLO_DATA_DEV"
 WAIT_SECS=30
 for i in $(seq 1 "$WAIT_SECS"); do
     [ -b "$KUBESOLO_DATA_DEV" ] && break
+    mdev -s 2>/dev/null || true
     sleep 1
 done