raspberry-pi: add structuredExtraConfig for vendor kernel

Override nixpkgs common-config.nix defaults that conflict with RPi
vendor defconfigs (bcmrpi/bcm2709/bcm2711/bcm2712):

- NR_CPUS: 4 (nixpkgs sets 384; RPi has 4 cores)
- CMA_SIZE_MBYTES: 5 (nixpkgs sets 32; vendor default is 5)
- PREEMPT/PREEMPT_VOLUNTARY: full preempt per vendor defconfig (Pi 3+ only;
  arm32 defconfigs use voluntary preempt which matches nixpkgs default)
- NFS root boot: NFS_FS, NFS_V4, ROOT_NFS, IP_PNP, IP_PNP_DHCP, IP_PNP_RARP
- NET_CLS_BPF, NLS_CODEPAGE_437: built-in instead of module
- FB_SIMPLE: simple framebuffer

Ref: https://github.com/raspberrypi/linux/tree/rpi-6.12.y/arch/arm64/configs

Author: JamieMagee

raspberry-pi/common/kernel.nix

@@ -50,6 +50,39 @@ lib.overrideDerivation
         request_key_helper
       ];
 
+      # Override nixpkgs common-config.nix defaults that conflict with the RPi vendor defconfigs.
+      # See: https://github.com/raspberrypi/linux/tree/rpi-6.12.y/arch/arm64/configs
+      structuredExtraConfig =
+        with lib.kernel;
+        {
+          # RPi has 4 cores; nixpkgs common-config sets 384
+          NR_CPUS = lib.mkForce (freeform "4");
+          # nixpkgs sets 32MB; RPi vendor defconfig uses 5MB
+          CMA_SIZE_MBYTES = lib.mkForce (freeform "5");
+
+          # NFS root boot support (common RPi use case)
+          NFS_FS = lib.mkForce yes;
+          NFS_V4 = lib.mkForce yes;
+          ROOT_NFS = yes;
+          IP_PNP = lib.mkForce yes;
+          IP_PNP_DHCP = yes;
+          IP_PNP_RARP = yes;
+
+          # Match vendor defconfig: built-in instead of module
+          NET_CLS_BPF = lib.mkForce yes;
+          NLS_CODEPAGE_437 = lib.mkForce yes;
+          FB_SIMPLE = yes;
+        }
+        # arm64 vendor defconfigs (bcm2711, bcm2712) use full preempt;
+        # arm32 ones (bcmrpi, bcm2709) use voluntary preempt (nixpkgs default)
+        // lib.optionalAttrs (rpiVersion >= 3) (
+          with lib.kernel;
+          {
+            PREEMPT = lib.mkForce yes;
+            PREEMPT_VOLUNTARY = lib.mkForce no;
+          }
+        );
+
       extraMeta =
         if (rpiVersion < 3) then
           {