On build-time retargeting

Controlix will eventually be able to retarget its build system, with the following major options:

• POSIX
• Simulated device drivers
• Native OS, real device drivers (for CPU-based system)
• Netlist generation (FPGAs or full IC synthesis)

This allows the bulk of the abstract control system to be coded and tested without having to mess with the time and space constraints of the lower-level interfaces when they aren't needed.