Refactor build system to use out-of-tree builds

[nchapman]

Problem

The current build system violates best practices by building components in-place within source directories. This causes several issues:

1. Toolchain changes leave stale artifacts - When switching GCC versions (e.g., GCC 13→10 for TG5050), old .o files persist and cause linking errors
2. make clean is fragile - Must enumerate every possible build artifact location (currently 615+ .o files across platform directories)
3. Source pollution - Build artifacts mixed with source files in workspace/*/other/, workspace/*/lib*/
4. Cross-platform conflicts - Different platforms building in same directories

Current Anti-pattern

workspace/tg5050/other/DinguxCommander-sdl2/
├── src/
│   ├── dialog.cpp
│   ├── dialog.o          ❌ Build artifact in source
│   └── main.o            ❌ Pollutes source tree
└── DinguxCommander       ❌ Binary in source directory

Impact

• Had to manually delete 615+ .o files when implementing TG5050 GCC 10.3 toolchain
• make clean was ineffective, requiring find commands to locate artifacts
• Easy to miss artifacts when adding new platforms/components

Proposed Solution

Implement out-of-tree builds where all build artifacts go into separate build/ tree:

build/
├── tg5050/
│   ├── intermediate/              # All .o files, temp builds
│   │   ├── launcher/
│   │   ├── player/
│   │   ├── DinguxCommander/
│   │   └── libmsettings/
│   └── SYSTEM/                    # Final outputs
│       └── bin/
└── miyoomini/
    └── intermediate/

Benefits:

• make clean becomes: rm -rf ./build
• Source directories stay clean
• No toolchain artifact conflicts
• Standard practice for build systems

Implementation Options

Option 1: Fix Platform Component Makefiles

Modify DinguxCommander, libmsettings, etc. makefiles to use BUILD_DIR variable and output to separate directories.

Pros: Proper solution, follows best practices
Cons: Requires maintaining patches for third-party components

Option 2: Docker Ephemeral Builds

Use Docker container filesystem as build scratch space, only copy final outputs to host.

Pros: Automatic cleanup, leverages existing Docker infrastructure
Cons: Requires refactoring build scripts

Option 3: Wrapper Makefiles

Create wrapper makefiles that intercept build and redirect outputs.

Pros: Minimal changes to third-party code
Cons: Additional complexity layer

Current Workaround

Temporary fix applied: Enhanced make clean to aggressively find/delete all .o, .elf files in workspace.

This is a band-aid solution that works but doesn't address root cause.

Acceptance Criteria

• All build artifacts go into build/ directory tree
• make clean is a single rm -rf ./build command
• Source directories contain zero build artifacts
• Works for all platforms (miyoomini, tg5050, etc.)
• Documented in build system docs

References

• GNU Make: Separating Source and Build Directories
• CMake: Out-of-source builds
• Kernel.org: kbuild out-of-tree