cmake: Fix trimming project path on Windows
cmake: Fix trimming project path on Windows

strings.TrimPrefix does not work properly if path already uses
backshasles as path separator, filepath.Rel will take care of that
automatically.

Also trimProjectPath should now replace backslashes internally to keep
returned value consistent.

apache/incubator-mynewt-newt,apache/incubator-mynewt-newt