image: T7818: avoid error when skipping config migration

Fix "No such file or directory" error triggered during image upgrade when user
opts not to migrate config.

GRUB cmdline migration incorrectly attempts to access non-existent config in
new image path.

Author: c-po

src/op_mode/image_installer.py

@@ -1135,11 +1135,12 @@ def add_image(image_path: str, vrf: str = None, username: str = '',
         if set_as_default:
             grub.set_default(image_name, root_dir)
 
-        cmdline_options = get_cli_kernel_options(
-            f'{target_config_dir}/config.boot')
-        grub_util.update_kernel_cmdline_options(' '.join(cmdline_options),
-                                                root_dir=root_dir,
-                                                version=image_name)
+        if Path(f'{target_config_dir}/config.boot').exists():
+            cmdline_options = get_cli_kernel_options(
+                f'{target_config_dir}/config.boot')
+            grub_util.update_kernel_cmdline_options(' '.join(cmdline_options),
+                                                    root_dir=root_dir,
+                                                    version=image_name)
 
     except OSError as e:
         # if no space error, remove image dir and cleanup