nixos/sdcard: make firmware partition offset configurable

Different boards using u-boot SPL require to write to different
locations. Sometimes, the 8MiB gap isn't sufficient - rk3399 boards
write to 0x16384 for example, which is at 8MiB, thus overriding the
fat32 partition with the SPL.

nixos/modules/installer/sd-card/sd-image.nix

@@ -55,6 +55,22 @@ in
       '';
     };
 
+    firmwarePartitionOffset = mkOption {
+      type = types.int;
+      default = 8;
+      description = ''
+        Gap in front of the /boot/firmware partition, in mebibytes (1024×1024
+        bytes).
+        Can be increased to make more space for boards requiring to dd u-boot
+        SPL before actual partitions.
+
+        Unless you are building your own images pre-configured with an
+        installed U-Boot, you can instead opt to delete the existing `FIRMWARE`
+        partition, which is used **only** for the Raspberry Pi family of
+        hardware.
+      '';
+    };
+
     firmwarePartitionID = mkOption {
       type = types.str;
       default = "0x2178694e";
@@ -177,7 +193,7 @@ in
         zstd -d --no-progress "${rootfsImage}" -o ./root-fs.img
 
         # Gap in front of the first partition, in MiB
-        gap=8
+        gap=${toString config.sdImage.firmwarePartitionOffset}
 
         # Create the image file sized to fit /boot/firmware and /, plus slack for the gap.
         rootSizeBlocks=$(du -B 512 --apparent-size ./root-fs.img | awk '{ print $1 }')