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', []):

recipe-generator/recipe_templates.yaml

@@ -60,6 +60,9 @@ templates:
         desktop:
            - fragments/packages/virtual-machine-manager/packages.ks
            - fragments/packages/virtual-machine-manager/post-scripts.ks
+      bootloader:
+         grub: fragments/bootloader/grub.ks
+         systemd-boot: fragments/bootloader/systemd-boot.ks
 
   # Live recipe - for live-desktop or live-server
   live: