NanoPC-T6: preserve HDMI baseline and force serial-getty baud handoff on USB-C debug UART

[Copilot]

Boot logs were visible on USB-C only during early kernel startup, then console output/login stopped; HDMI had to remain on the previously working baseline. This change targets post-boot serial handoff instead of further kernel/console-device churn.

• USB-C serial login handoff

  • Override serial-getty@ttyS2 and serial-getty@ttyS0 ExecStart to explicitly keep high-speed UART negotiation.
  • Configure agetty with --keep-baud 1500000,115200,57600,38400,9600 so runtime getty can continue at the early boot speed used by RK3588 debug UART.

• HDMI-preserving scope

  • Keep NanoPC-T6 kernel/console baseline aligned with the rollback state that preserves HDMI behavior.
  • Avoid additional ttyFIQ0/kernel flavor expansion in this iteration.

systemd.services."serial-getty@ttyS2".serviceConfig.ExecStart = lib.mkForce [
  ""
  "${pkgs.util-linux}/bin/agetty -o '-p -- \\\\u' --keep-baud 1500000,115200,57600,38400,9600 ttyS2 $TERM"
];

systemd.services."serial-getty@ttyS0".serviceConfig.ExecStart = lib.mkForce [
  ""
  "${pkgs.util-linux}/bin/agetty -o '-p -- \\\\u' --keep-baud 1500000,115200,57600,38400,9600 ttyS0 $TERM"
];

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