Refactor build system to make configuration switching easier

[mwpenny]

What is the enhancement?

The Makefile is very coupled to the (libultra-based) Modern SDK. This issue tracks the work of refactoring it so it is easier to switch the toolchain and SDK at build time. There is also an opportunity to fix dependencies.

This will simplify the work of supporting other SDKs (and even potentially other platforms). CMake could be worth using here.