Clean up and update build process
1. Make use of hdlmake v2.1
2. For git modules in Manifest.py, point to specific git commits and
not at branch heads
3. remove and (git)ignore all hdlmake-generated files
1. Make use of hdlmake v2.1
2. For git modules in Manifest.py, point to specific git commits and
not at branch heads
3. remove and (git)ignore all hdlmake-generated files