Added vars for FindBinutils

Added setting of _CMAKE_TOOLCHAIN_PREFIX and _CMAKE_TOOLCHAIN_LOCATION
removed <...>/bin from CMAKE_SYSTEM_PREFIX_PATH

I think that <...>/bin isn't required for system prefix path. When I run without this change I get binutils from /usr/ instead of ARDUINO_SDK_PATH
Merge pull request #85 from cheshirekow/master

Added vars for FindBinutils