Added support for Microsemi Libero SoC

To synthesis and build bitstreams for Microsemmi PolarFire, RTG4, IGLOO2 and SmartFusion2 FPGAs the relatively new Libero SoC software has to be used. As there was not yet support for this in hdlmake, it was created. This work is based on the already existing script (libero.py). As the TCL command set was changed by Microsemi, the scripts were adjusted to the new commands.

The user can now choose between syn_tool = "libero" (for Libero IDE) or syn_tool = "liberosoc" (for Libero SoC).