Extract external toolchain build to mk/external/binutils.mk

[pbalduino]

Parent Issue

Part of #352 - Makefile refactoring

Objective

Dedicate a file to the toolchain rebuild flow and related variables.

Scope

Extract from root Makefile:

• Lines ~124: Toolchain-related variables
• Lines ~994: Toolchain rebuild flow (binutils, etc.)
• External dependency build rules
• Vendor toolchain configuration

Target Location

mk/external/binutils.mk

Implementation Steps

1. Create mk/external/ directory structure
2. Extract binutils-related variables
3. Move toolchain rebuild rules
4. Update root Makefile to include mk/external/binutils.mk
5. Test toolchain rebuild if applicable
6. Verify vendored toolchain configuration

Dependencies

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

Acceptance Criteria

• [ ] All toolchain build logic in mk/external/binutils.mk
• [ ] Toolchain rebuild works correctly
• [ ] Binutils configuration clear and accessible
• [ ] Easy to add new external dependencies
• [ ] No behavioral changes