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>

build/config/modules.list

@@ -1,30 +1,78 @@
 # Kernel modules loaded at boot by init
 # One module per line. Lines starting with # are ignored.
-# Modules are loaded in order listed.
+# Modules are loaded in order listed — dependencies must come first.
 
-# Networking — bridge and netfilter (required for K8s pod networking)
-br_netfilter
-bridge
-veth
-vxlan
+# Network device drivers (loaded early so interfaces are available)
+e1000
+e1000e
+virtio_net
 
-# Netfilter / iptables (required for kube-proxy and service routing)
-ip_tables
-iptable_nat
-iptable_filter
-iptable_mangle
-nf_nat
-nf_conntrack
-nf_conntrack_netlink
+# Virtio support (for VMs — block, entropy)
+virtio_blk
+virtio_rng
 
 # Filesystem — overlay (required for containerd)
 overlay
 
-# Conntrack (required for K8s services)
-nf_conntrack
+# Netfilter dependencies (must load before conntrack)
+nf_defrag_ipv4
+nf_defrag_ipv6
 
-# Optional — useful for CNI plugins and diagnostics
-tun
+# Netfilter / connection tracking (required for kube-proxy)
+nf_conntrack
+nf_nat
+nf_conntrack_netlink
+
+# nftables (modern iptables backend — kernel 6.18 uses nf_tables, not ip_tables)
+nf_tables
+nft_compat
+nft_chain_nat
+nft_ct
+nft_masq
+nft_nat
+nft_redir
+
+# Netfilter xt match/target modules (used by kube-proxy iptables rules via nft_compat)
+xt_conntrack
+xt_MASQUERADE
+xt_mark
+xt_comment
+xt_multiport
+xt_nat
+xt_addrtype
+xt_connmark
+xt_REDIRECT
+xt_recent
+xt_statistic
+xt_set
+
+# nft extras (reject, fib — used by kube-proxy nf_tables rules)
+nft_reject
+nft_reject_ipv4
+nft_reject_ipv6
+nft_fib
+nft_fib_ipv4
+nft_fib_ipv6
+
+# Reject targets (used by kube-proxy iptables-restore rules)
+nf_reject_ipv4
+nf_reject_ipv6
+ipt_REJECT
+ip6t_REJECT
+
+# nfacct extension (kube-proxy probes for it)
+xt_nfacct
+
+# Networking — bridge and netfilter (required for K8s pod networking)
+# Load order: llc → stp → bridge → br_netfilter
+llc
+stp
+bridge
+br_netfilter
+veth
+vxlan
+
+# IPVS — useful for kube-proxy IPVS mode and CNI plugins
 ip_vs
 ip_vs_rr
 ip_vs_wrr