Add flow Yosys + nextpnr + ???

[mithro]

This depends on #1

• Yosys (thus ABC?)
• nextpnr
• icestorm
• Project Trellis
• Project Oxide

[umarcor]

Refs:

• hdl.github.io/awesome/items/yosys
• hdl.github.io/awesome/items/nextpnr
• hdl.github.io/awesome/items/icestorm
• hdl.github.io/awesome/items/prjtrellis
• hdl.github.io/awesome/items/prjoxide
• hdl.github.io/awesome/items/mistral

[per-gron]

There are rules for

• nextpnr https://github.com/google/xls/tree/main/dependency_support/nextpnr
• icestorm https://github.com/google/xls/tree/main/dependency_support/at_clifford_icestorm
• prjtrellis https://github.com/google/xls/tree/main/dependency_support/prjtrellis https://github.com/google/xls/tree/main/dependency_support/prjtrellis_db

[per-gron]

Here is a C++ program in XLS that calls Yosys and Nextpnr to synthesise verilog code for Lattice FPGAs: https://github.com/google/xls/blob/main/xls/synthesis/yosys/yosys_server_main.cc