Overhaul Build System

[bangcorrupt]

Build system has outgrown itself and requires overhaul.

User code should be in a separate tree, under a single project root directory.

cpu/src/common is copied to dsp/src/common during build. This should be moved up to the top level and given a separate makefile for each processor.

bfin_ldr.h is generated during build and copied to cpu/src/resources. We don't want to track the file, but the directory needs to exist else build fails. Build system should not be modifying files in the source tree. Currently we have an empty file to let git track the empty directory. This is jank.

I'm also getting different results building in my original repo compared with the repo cloned from github. Both binaries appear to work correctly, but are a different size. All of the submodules are on the same commit, using the same compiler versions with the same optimisation flags. I have no idea why this is happening.