Re-write CMakeLists.txt away from custom cmds

These changes are deeper than the PiEmu changes, but then that's embedded targets for you! This allows compilation on Windows. Mainly the only *required* change was to find the Python Interpreter. But I went ahead and moved everything to work off of a toolchain file, which is the more correct way of using CMake for embedded projects.

Please review and let me know what you think. I will test on Linux to make sure everything's okay. More toolchain files can easily be added.

* Introduce toolchain file instead of trying to search for any arm compiler (although any *would* do)
* Set project as assembler project - NOTE: CMake still uses gcc to assemble, hence we need to instruct it when linking not to use stdlibs or startfiles (crt0, etc.)
* Properly search for python interpreter. Because of the toolchain file python must be present in your PATH env var

Tested against running PiFox with output kernel image on Windows 7 using the arm-none-eabi- toolchain (sorry commit message states wrong compiler!)

Allows compilation under Windows. Untested on Linux but //should// be fine. Will test ASAP.