Extract kernel build logic to mk/kernel/build.mk

[pbalduino]

Parent Issue

Part of #352 - Makefile refactoring

Objective

Hold kernel object discovery, build flags, and the heavy build recipe with Limine/image creation in a dedicated kernel build file.

Scope

Extract from root Makefile:

• Lines ~61: Kernel source/object discovery
• Lines ~576: Kernel build flags
• Lines ~635: Limine/image creation recipe
• Kernel-specific compilation rules

Target Location

mk/kernel/build.mk

Implementation Steps

1. Create mk/kernel/ directory structure
2. Extract kernel object patterns and discovery logic
3. Move kernel-specific build flags
4. Extract Limine integration and image creation
5. Update root Makefile to include mk/kernel/build.mk
6. Test make kernel or equivalent target
7. Verify bootable image is created correctly

Dependencies

• #353 (core config)
• #354 (core docker)

Acceptance Criteria

• [ ] All kernel build logic in mk/kernel/build.mk
• [ ] Kernel builds successfully
• [ ] Bootable image created correctly
• [ ] Limine integration works
• [ ] No behavioral changes