Fix NanoPC-T6 reset button: merge kernel config and apply patches directly in kernel override

[Copilot]

Reset button works in U-Boot but stops working once the Linux kernel starts. After extensive iteration (7+ attempts), two Nix-level issues were identified that prevented kernel patches from taking effect reliably.

NanoPC-T6 button hardware (from schematic)

Button	Connection	Mechanism
Power	RK806 PMIC PWRON	Software — KEY_POWER via rk805-pwrkey driver
Reset	RK806 PMIC RESETB	Hardware — PMIC register-configured, no kernel event
Mask ROM	SARADC ch0	Software — adc-keys driver

Changes

• structuredExtraConfig was replaced, not merged — set-form override { structuredExtraConfig = ... } silently dropped nabam's base rockchip config (REGULATOR_RK808, GPIO_ROCKCHIP, SPI_ROCKCHIP, etc.). Switched to function-form override:

customKernel = baseKernel.kernel.override (prev: {
  structuredExtraConfig = (prev.structuredExtraConfig or {}) // (with lib.kernel; {
    MFD_RK8XX_SPI = yes;
    PINCTRL_RK805 = yes;
    INPUT_RK805_PWRKEY = yes;
  });
  kernelPatches = (prev.kernelPatches or []) ++ [ ... ];
});

• Patches moved from boot.kernelPatches into kernel override — boot.kernelPatches relies on the NixOS module apply function to propagate via .extend. Including patches directly in kernelPatches arg is more direct and survives lib.mkForce on boot.kernelPackages.

• DTS patch adds rockchip,reset-mode = <2> to configure PMIC reset behavior when RESETB is asserted.

• Kernel driver patch disables SLAVE_RESTART_FUN in rk8xx-core.c pre-init — this multi-PMIC feature repurposes RESETB input on the single-PMIC NanoPC-T6.

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