_toolchain,build.plat,vendor.*: add required_tools list and checks.

nmigen/vendor/lattice_ecp5.py

@@ -239,6 +239,14 @@ class LatticeECP5Platform(TemplatedPlatform):
         assert toolchain in ("Trellis", "Diamond")
         self.toolchain = toolchain
 
+    @property
+    def required_tools(self):
+        if self.toolchain == "Trellis":
+            return ["yosys", "nextpnr-ecp5", "ecppack"]
+        if self.toolchain == "Diamond":
+            return ["pnmainc", "ddtcmd"]
+        assert False
+
     @property
     def file_templates(self):
         if self.toolchain == "Trellis":

nmigen/vendor/lattice_ice40.py

@@ -39,6 +39,12 @@ class LatticeICE40Platform(TemplatedPlatform):
     device  = abstractproperty()
     package = abstractproperty()
 
+    required_tools = [
+        "yosys",
+        "nextpnr-ice40",
+        "icepack",
+    ]
+
     _nextpnr_device_options = {
         "iCE40LP384": "--lp384",
         "iCE40LP1K":  "--lp1k",

nmigen/vendor/xilinx_7series.py

@@ -50,6 +50,8 @@ class Xilinx7SeriesPlatform(TemplatedPlatform):
     package = abstractproperty()
     speed   = abstractproperty()
 
+    required_tools = ["vivado"]
+
     file_templates = {
         **TemplatedPlatform.build_script_templates,
         "{{name}}.v": r"""

nmigen/vendor/xilinx_spartan_3_6.py

@@ -57,6 +57,14 @@ class XilinxSpartan3Or6Platform(TemplatedPlatform):
     package = abstractproperty()
     speed   = abstractproperty()
 
+    required_tools = [
+        "xst",
+        "ngdbuild",
+        "map",
+        "par",
+        "bitgen",
+    ]
+
     @property
     def family(self):
         device = self.device.upper()