[Fletchgen] Specifing the -o option, breaks top level generation

[EraYaN]

Describe the bug
If the -o option is given, the AxiTop and SimTop_tc files are not created. If The path is hardcoded in fletchgen.cc@L104 from std::string axi_file_path = "vhdl/AxiTop.vhd"; to std::string axi_file_path = "/output/vhdl/AxiTop.vhd"; it does work. It also works without the -o option and saving to the current directory.

To Reproduce

fletchgen -o /output -i /source/ff_in.fbs /source/ff_out.fbs -l vhdl --axi --sim -r /source/Float_data.rb -s /source/Float_data.srec --force -n FletcherFloat

vs

cd /output && fletchgen -i /source/ff_in.fbs /source/ff_out.fbs -l vhdl --axi --sim -r /source/Float_data.rb -s /source/Float_data.srec --force -n FletcherFloat

Expected behavior
Both should result in the same output.