sd-image: add preBuildCommands option (#501160)

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

@@ -179,12 +179,44 @@ in
       '';
     };
 
+    preBuildCommands = mkOption {
+      type = types.lines;
+      example = literalExpression ''
+        '''
+          if [ ! -w "$root_fs" ]; then
+            cp --no-preserve=mode "$root_fs" ./root-fs.img
+            root_fs=./root-fs.img
+          fi
+          resize2fs $root_fs 15G
+        '''
+      '';
+      default = "";
+      description = ''
+        Shell commands to run after the root filesystem image has been
+        prepared, but before the final SD image is assembled.
+
+        The path to the root filesystem image is available in the
+        {var}`root_fs` shell variable. This hook can be used to modify
+        the rootfs, for example to resize it or inject additional files.
+
+        Note that when {option}`sdImage.compressImage` is disabled,
+        {var}`root_fs` points to a read-only store path. To modify it,
+        first copy it to a writable location and update {var}`root_fs`
+        to point to the copy.
+      '';
+    };
+
     postBuildCommands = mkOption {
+      type = types.lines;
       example = literalExpression "'' dd if=\${pkgs.myBootLoader}/SPL of=$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.
+        Shell commands to run after the SD image has been assembled.
+
+        The path to the image is available in the {var}`img` shell
+        variable. This hook is typically used for boards that require
+        writing a bootloader (such as u-boot SPL) to a fixed offset
+        before the first partition.
       '';
     };
 
@@ -284,6 +316,8 @@ in
             zstd -d --no-progress "${config.sdImage.rootFilesystemImage}" -o $root_fs
           ''}
 
+          ${config.sdImage.preBuildCommands}
+
           # Gap in front of the first partition, in MiB
           gap=${toString config.sdImage.firmwarePartitionOffset}