Extract environment and toolchain configuration to mk/core/config.mk

[pbalduino]

Parent Issue

Part of #352 - Makefile refactoring

Objective

Move environment/toolchain detection and shared flags to a dedicated configuration file.

Scope

Extract from root Makefile:

• Lines ~1: Initial environment setup
• Lines ~175: Toolchain configuration
• Lines ~203: Shared compiler/linker flags
• Lines ~263: Helper macros like assert_tools

Target Location

mk/core/config.mk

Implementation Steps

1. Create mk/core/ directory structure
2. Extract configuration variables and macros
3. Update root Makefile to include mk/core/config.mk
4. Verify all targets still work: make clean && make all
5. Test Docker and host builds if applicable

Dependencies

None - this is a foundational module

Acceptance Criteria

• [ ] All environment/toolchain variables moved to mk/core/config.mk
• [ ] Root Makefile includes the new file
• [ ] All existing make targets work unchanged
• [ ] No behavioral changes to builds