vendor: yosys is a required tool for all Verilog-based flows.

nmigen/vendor/lattice_ecp5.py

@@ -165,6 +165,7 @@ class LatticeECP5Platform(TemplatedPlatform):
     # Diamond templates
 
     _diamond_required_tools = [
+        "yosys",
         "pnmainc",
         "ddtcmd"
     ]

nmigen/vendor/lattice_ice40.py

@@ -170,6 +170,7 @@ class LatticeICE40Platform(TemplatedPlatform):
     # iCECube2 templates
 
     _icecube2_required_tools = [
+        "yosys",
         "synthesis",
         "synpwrap",
         "tclsh",

nmigen/vendor/xilinx_7series.py

@@ -51,7 +51,10 @@ class Xilinx7SeriesPlatform(TemplatedPlatform):
     package = abstractproperty()
     speed   = abstractproperty()
 
-    required_tools = ["vivado"]
+    required_tools = [
+        "yosys",
+        "vivado"
+    ]
 
     file_templates = {
         **TemplatedPlatform.build_script_templates,

nmigen/vendor/xilinx_spartan_3_6.py

@@ -59,6 +59,7 @@ class XilinxSpartan3Or6Platform(TemplatedPlatform):
     speed   = abstractproperty()
 
     required_tools = [
+        "yosys",
         "xst",
         "ngdbuild",
         "map",