feat(extensions): add netboot extension for TFTP+NFS diskless boot

ROOTFS_TYPE=nfs alone only gives a hybrid image: kernel+DTB still live
on local storage, only / comes over NFS. The netboot extension takes it
further: kernel, DTB, optional uInitrd and a PXE/extlinux config are
staged under output/images/<version>-netboot-tftp/ ready to drop into
a TFTP root, and a matching NFS rootfs tree/archive is produced from
the same build. The target then needs nothing but DHCP+TFTP+NFS to boot.

extensions/netboot/netboot.sh hooks:
- extension_prepare_config: force ROOTFS_TYPE=nfs, validate vars,
  compute defaults for NETBOOT_TFTP_PREFIX / NETBOOT_NFS_PATH (shared
  by LINUXFAMILY/BOARD/BRANCH/RELEASE, or per-host when
  NETBOOT_HOSTNAME is set), normalize NETBOOT_CLIENT_MAC.
- custom_kernel_config: enable ROOT_NFS, NFS_FS, NFS_V3, IP_PNP,
  IP_PNP_DHCP so root=/dev/nfs ip=dhcp works without initrd.
- post_customize_image: drop armbian-resize-filesystem.service
  (meaningless on NFS root) and /root/.not_logged_in_yet (the
  armbian-firstlogin whiptail wizard blocks bring-up when there is
  no interactive console). armbian-firstrun.service stays — it only
  regenerates SSH host keys.
- host_pre_docker_launch: bind-mount ROOTFS_EXPORT_DIR into the build
  container when it lives outside ${SRC}, so the single-step
  builder-as-NFS-server workflow works the same in and out of Docker.
- pre_umount_final_image: assemble the TFTP tree (Image/zImage, dtb/,
  uInitrd), write pxelinux.cfg/{default.example | 01-<mac>} with the
  right FDT/FDTDIR line, and expose a netboot_artifacts_ready hook
  for userpatches that deploy to a real server.

lib/functions/image/rootfs-to-image.sh:
- ROOTFS_COMPRESSION: gzip (default) | zstd | none. Produces
  .tar.gz/.tar.zst or skips the archive entirely.
- ROOTFS_EXPORT_DIR: when set, additionally rsync the rootfs tree into
  that directory. Combined with ROOTFS_COMPRESSION=none on an NFS-
  server build host this turns netboot deployment into a single build
  step with no tar/unpack/transport phase.
- ROOTFS_ARCHIVE_PATH is exported so downstream hooks
  (netboot_artifacts_ready) can reference the produced archive.

No PXE config is written when NETBOOT_SERVER is empty: nfsroot= keeps
${serverip} literal so U-Boot expands it at `pxe boot` from DHCP
siaddr. Intentionally no console= in APPEND — hardcoding a baud
breaks boards like helios64 (1500000); the kernel picks up the
console from DTB /chosen/stdout-path, earlycon keeps early output.

Validated end-to-end on helios64 (Armbian 26.05 edge/resolute):
both tree-workflow (ROOTFS_COMPRESSION=none + ROOTFS_EXPORT_DIR) and
archive-workflow builds boot cleanly over TFTP+NFS to a login prompt
with no whiptail wizard and no resize2fs errors.

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

Author: iav