quartus: add support for qsys generation

When the (new) 'qsys_files' entry under the 'quartus' section is
present, a shell script is created with the necessary commands
generate a system described in a .qsys file.
The approach with first creating a shell script and then
executing that, instead of executing the commands directly from
fusesoc was chosen to keep the build directory still completely
independent from fusesoc once generated.