vendor.xilinx_spartan_3_6: set bitgen defaults to -g Binary:Yes -g Compress.

* `-g Binary:Yes` should be overridable.
  * `-g Compress` is a good default.

nmigen/vendor/lattice_ice40.py

@@ -24,8 +24,8 @@ class LatticeICE40Platform(TemplatedPlatform):
         * ``script_after_read``: inserts commands after ``read_ilang`` in Yosys script.
         * ``script_after_synth``: inserts commands after ``synth_ice40`` in Yosys script.
         * ``yosys_opts``: adds extra options for Yosys.
-        * ``nextpnr_opts``: adds extra and overrides default options (``--placer heap``)
-          for nextpnr.
+        * ``nextpnr_opts``: adds extra and overrides default options for nextpnr;
+          default options: ``--placer heap``.
 
     Build products:
         * ``{{name}}.rpt``: Yosys log.

nmigen/vendor/xilinx_spartan_3_6.py

@@ -154,9 +154,8 @@ class XilinxSpartan3Or6Platform(TemplatedPlatform):
         """,
         r"""
         {{get_tool("bitgen")}}
-            {{get_override("bitgen_opts")|default([])|options}}
+            {{get_override("bitgen_opts")|default(["-g Binary:Yes", "-g Compress"])|options}}
             -w
-            -g Binary:Yes
             {{name}}_par.ncd
             {{name}}.bit
         """