build system: upgrade build system, eliminate differences in core definitions.
    
    Prior to this patch, there were several differences between this project's
    build system and the one from which it was derived (sofort). Although the
    differences were very minor and for the most part related to this project
    being part of a free-standing, midipix-specific development framework,
    they still added an extra maintenance burden, specifically by requiring
    that common changes be applied via patch(1) rather than git-am(1).
    
    Following recent improvements to the common build system, it is now
    possible to have a free-standing, midipix-specific project without
    any changes to the core build system files, hence the current upgrade.