Generate rebar.config file from rebar.config.script

This may be needed if you use a tool that expects a static rebar config file (for example Intellij).

Here is how to temporarily generate the file:

1> {ok, Data} = file:script("rebar.config.script").
...
2> file:write_file("rebar.config", io_lib:fwrite("~p.\n", [Data])).
ok