Generate resulting .cpp in the $BUILD_DIR, no need for atexit
Register middleware only once, there's no need to manually walk the
project directory since fnmatch.fnmatch(...) supports globs
Provide a SCons Action instead of simple function, so there's a way to
generate things on demand via command line
```
$ pio run -e $env -t .pio/build/$env/espurna_single_source/src/main.cpp
```