nixos/sdImage: add postBuildCommands

This allows to perform `dd if= of=$img` after the image is built
which is handy to add e.g. uBoot SPL to the built image.

Instructions for some ARM boards sometimes contain this step
that needs to be performed manually, with this patch it can be
part of the nix file used to built the image.

nixos/modules/installer/cd-dvd/sd-image.nix

@@ -108,6 +108,15 @@ in
       '';
     };
 
+    postBuildCommands = mkOption {
+      example = literalExample "'' dd if=\${pkgs.myBootLoader}/SPL of=$img bs=1024 seek=1 conv=notrunc ''";
+      default = "";
+      description = ''
+        Shell commands to run after the image is built.
+        Can be used for boards requiring to dd u-boot SPL before actual partitions.
+      '';
+    };
+
     compressImage = mkOption {
       type = types.bool;
       default = true;
@@ -197,6 +206,9 @@ in
         # Verify the FAT partition before copying it.
         fsck.vfat -vn firmware_part.img
         dd conv=notrunc if=firmware_part.img of=$img seek=$START count=$SECTORS
+
+        ${config.sdImage.postBuildCommands}
+
         if test -n "$compressImage"; then
             zstd -T$NIX_BUILD_CORES --rm $img
         fi