Ignition: tweak Target synthesis parameters to improve P&R

BUILD.conf

@@ -117,16 +117,23 @@ environment('icestick', base = 'ice40', contents = {
     ],
     'nextpnr_constraints': ROOT + '/hdl/projects/icestick/icestick.pcf',
 })
-environment('ignition_target', base = 'ice40', contents = {
+environment('ignition_target', base = 'default', contents = {
     'bsc_flags': [
         '-opt-undetermined-vals',
         '-unspecified-to', 'X',
     ],
+    'yosys_cmds': [
+        'synth_ice40 -top $$top_module -dffe_min_ce_use 4',
+    ],
+    'yosys_backend': 'json', # nextpnr assumes JSON input.
+    'nextpnr_ice40': VARS.get('nextpnr', 'ice40', default='nextpnr-ice40'),
     'nextpnr_ice40_flags': [
+        '-q',
         '--lp1k',
         '--package qn84',
         '--freq 50',
     ],
+    'nextpnr_ice40_pack': VARS.get('nextpnr', 'ice40_pack', default='icepack'),
     'nextpnr_constraints': ROOT + '/hdl/ip/bsv/ignition/ignition_target.pcf',
 })
 environment('gimlet', base = 'ice40', contents = {