Add bootloader modifier to install template

- Add bootloader modifier following security pattern (override, not additive)
- Supports 'grub' and 'systemd-boot' options
- Default uses grub from required, override via modifier

Closes #issue

recipe-generator/generate_recipe.py

@@ -244,7 +244,7 @@ class RecipeGenerator:
         modifiers['version'] = version
 
         # Keys that can be overridden by modifiers
-        override_keys = {'security'}
+        override_keys = {'security', 'bootloader'}
 
         # Add required includes
         for item in template.get('required', []):