build: separate generic ARM64 from Raspberry Pi kernel builds

Splits the ARM64 build into two tracks per docs/arm64-architecture.md:

Generic ARM64 (mainline kernel.org, UEFI, virtio, GRUB):
- New build/scripts/build-kernel-arm64.sh builds mainline LTS (6.12.x by default)
  from arm64 defconfig + shared container fragment + arm64-virt enables
  (VIRTIO_*, EFI_STUB, NVMe). Output: build/cache/kernel-arm64-generic/.
- New Makefile targets: kernel-arm64, rootfs-arm64 (now consumes the mainline
  kernel modules via TARGET_VARIANT=generic).
- versions.env: pin MAINLINE_KERNEL_VERSION=6.12.10, declare cdn.kernel.org URL
  and SHA256 placeholder.

Raspberry Pi (raspberrypi/linux fork, custom DTBs, autoboot.txt):
- build-kernel-arm64.sh (RPi-flavoured) renamed to build-kernel-rpi.sh; cache
  dir renamed from custom-kernel-arm64 to custom-kernel-rpi.
- New Makefile targets: kernel-rpi, rootfs-arm64-rpi (uses TARGET_VARIANT=rpi).
- rpi-image now depends on rootfs-arm64-rpi + kernel-rpi instead of the generic
  rootfs-arm64.
- create-rpi-image.sh + inject-kubesolo.sh updated to reference the new cache
  path. inject-kubesolo.sh now takes a TARGET_VARIANT env var (rpi|generic) to
  select which ARM64 kernel modules to consume.

Shared substrate:
- rpi-kernel-config.fragment renamed to kernel-container.fragment. The contents
  were never RPi-specific (cgroup, namespaces, AppArmor, netfilter) — just
  misnamed. Extended with extra subsystem disables (KVM, WLAN, CFG80211,
  INFINIBAND, PCMCIA, HAMRADIO, ISDN, ATM, INPUT_JOYSTICK, INPUT_TABLET, FPGA)
  and CONFIG_LSM=lockdown,yama,apparmor.
- build-kernel.sh (x86) refactored to apply the shared fragment via a generic
  apply_fragment function (two-pass for the TC stock config security dance),
  killing ~50 lines of inline config duplication.

Note: rename detection shows build-kernel-arm64.sh as 'modified' because the
new file at that path is the mainline build, while the old RPi-flavoured
content lives in build-kernel-rpi.sh (which appears as a new file). The git
log for build-kernel-rpi.sh is empty; the RPi history is preserved at the
original path until this commit.

No actual kernel build runs in this commit — that's Phase 3 work.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

build/config/kernel-container.fragment

@@ -0,0 +1,90 @@
+# KubeSolo OS — Shared kernel config fragment for container workloads
+#
+# Applied on top of:
+#   - Tiny Core stock config (x86_64)        via build-kernel.sh
+#   - mainline kernel.org arm64 defconfig    via build-kernel-arm64.sh
+#   - bcm2711_defconfig / bcm2712_defconfig  via build-kernel-rpi.sh
+#
+# All entries here are architecture-agnostic.
+# Apply this fragment twice with `make olddefconfig` between passes — TC's stock
+# config has CONFIG_SECURITY disabled, which causes a single-pass olddefconfig
+# to strip the security subtree before its dependencies (SYSFS, MULTIUSER) are
+# resolved.
+
+# cgroup v2 (mandatory for containerd/runc)
+CONFIG_CGROUPS=y
+CONFIG_CGROUP_CPUACCT=y
+CONFIG_CGROUP_DEVICE=y
+CONFIG_CGROUP_FREEZER=y
+CONFIG_CGROUP_SCHED=y
+CONFIG_CGROUP_PIDS=y
+CONFIG_MEMCG=y
+CONFIG_CGROUP_BPF=y
+CONFIG_CFS_BANDWIDTH=y
+
+# BPF (required for cgroup v2 device control)
+CONFIG_BPF=y
+CONFIG_BPF_SYSCALL=y
+
+# Namespaces (mandatory for containers)
+CONFIG_NAMESPACES=y
+CONFIG_NET_NS=y
+CONFIG_PID_NS=y
+CONFIG_USER_NS=y
+CONFIG_UTS_NS=y
+CONFIG_IPC_NS=y
+
+# Device management
+CONFIG_DEVTMPFS=y
+CONFIG_DEVTMPFS_MOUNT=y
+
+# Filesystem
+CONFIG_OVERLAY_FS=y
+CONFIG_SQUASHFS=y
+CONFIG_EXT4_FS=y
+CONFIG_VFAT_FS=y
+
+# Networking
+CONFIG_BRIDGE=m
+CONFIG_NETFILTER=y
+CONFIG_NF_CONNTRACK=m
+CONFIG_NF_NAT=m
+CONFIG_NF_TABLES=m
+CONFIG_VETH=m
+CONFIG_VXLAN=m
+
+# Security: AppArmor + Audit
+CONFIG_AUDIT=y
+CONFIG_AUDITSYSCALL=y
+CONFIG_SECURITY=y
+CONFIG_SECURITYFS=y
+CONFIG_SECURITY_NETWORK=y
+CONFIG_SECURITY_APPARMOR=y
+CONFIG_DEFAULT_SECURITY_APPARMOR=y
+CONFIG_LSM=lockdown,yama,apparmor
+
+# Security: seccomp
+CONFIG_SECCOMP=y
+CONFIG_SECCOMP_FILTER=y
+
+# Crypto (image verification)
+CONFIG_CRYPTO_SHA256=y
+
+# Disable unnecessary subsystems for headless edge appliance
+# CONFIG_SOUND is not set
+# CONFIG_DRM is not set
+# CONFIG_KVM is not set
+# CONFIG_MEDIA_SUPPORT is not set
+# CONFIG_WIRELESS is not set
+# CONFIG_WLAN is not set
+# CONFIG_CFG80211 is not set
+# CONFIG_BT is not set
+# CONFIG_NFC is not set
+# CONFIG_INFINIBAND is not set
+# CONFIG_PCMCIA is not set
+# CONFIG_HAMRADIO is not set
+# CONFIG_ISDN is not set
+# CONFIG_ATM is not set
+# CONFIG_INPUT_JOYSTICK is not set
+# CONFIG_INPUT_TABLET is not set
+# CONFIG_FPGA is not set