- Added configuration options for SDK location and version.
- Separated source tracking variables into application C sources, SDK C sources, SDK ASM sources and
and SDK headers (the latter were added so they can be downloaded individually).
- Added automated prerequisite generation for SDK object file rules. This compensates for the fact
that chains of implicit rules cannot work anymore as they did, as file location can't be
determined through vpath anymore (downloaded files don't exist until after rules are procesed).