To do: move build_dir_sim and build_dir_synth from hdllib.cfg to hdltool.cfg.

tools/hdltool_readme.txt

@@ -577,7 +577,6 @@ f) Generate Quartus IP key
    - Using build_dir_synth is also sufficient/suitable to define the build subdirectory for IP generation.
    
 g) hdltool.cfg per toolset
-   . Is the hdltool.cfg necessary/useful at all?
    . The model_tech_dir key in hdltool.cfg depends on the modelsim version that is defined in the toolset. The
      model_tech_dir gives the path to the modelsim.ini that needs to be included in the mpf, because without the
      IEEE and STD libraries are not known anymore after the mpf is loaded.
@@ -596,9 +595,13 @@ g) hdltool.cfg per toolset
        model_tech_dir = /home/software/Mentor/10.4/modeltech
    
      Currently the 6.6c modelsim.ini is used even if Modelsim 10.4 is ran.
+   . The build_dir_sim and build_dir_synth are defined in the hdllib.cfg and both set to $HDL_BUILD_DIR. It seems
+     better to define these keys in the hdltool.cfg, because currently all they are thesame in all hdllib.cfg and
+     with the possibility to define different hdltool.cfg there is already enough freedom.
      
    Conclusion:
-   - Keep hdltool.cfg, but define dedicated hdltool_<toolset>.cfg and add --toolset command line option.
+   - Is the hdltool.cfg necessary/useful at all? Yes. Keep hdltool.cfg, but define dedicated hdltool_<toolset>.cfg
+     and add --toolset command line option.
 
 h) regression test script
    - To simulate all self-checking VHDL test benches and report the result.