Move NanoPC‑T6 back to nabam kernel path and adopt latest Rockchip variant

[Copilot]

NanoPC‑T6 showed better reset/bring-up behavior on the FriendlyARM fork. This change keeps the build on the nabam fork while selecting its newer Rockchip kernel line to better match those board-level fixes.

• Kernel source strategy

  • Switched NanoPC‑T6 kernel selection from direct FriendlyARM source override to nabam’s Rockchip package set.
  • Updated boot.kernelPackages to use kernel_linux_latest_rockchip_unstable from inputs.rockchip.legacyPackages.

• Cleanup

  • Removed the temporary board-local FriendlyARM kernel derivation (nix/builders/nanopc-t6/kernel-friendlyarm.nix).

• Resulting behavior target

  • Preserve the nabam integration path while pulling in newer Rockchip kernel behavior that is closer to what worked with FriendlyARM.

# nix/builders/nanopc-t6/base.nix
boot.kernelPackages =
  inputs.rockchip.legacyPackages.aarch64-linux.kernel_linux_latest_rockchip_unstable;

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