Add new build system for Ivory modules.

- Combine "flight-support" and "flight-generated" into "src/flight".
- Add "ivory.mk" with the "ivory_pkg" macro to handle Ivory code
  generation.
- Update all modules that use Ivory-generated code use the new
  "ivory_pkg" macro.
- Put all Ivory-generated src in "build/gen", not in the src dir.
- Move Ivory LED test into the "apps" directory, like the UART test.
- Delete build directory on "make clean".