Re-write CMakeLists.txt away from custom cmds

* 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 executable on Windows 7 using
mingw-w64 toolchain

Allows compilation with MinGW (Windows). Untested on Linux but
//should// be fine. Will test ASAP.