Clean up kicad backup stuff in mini board directory

hardware/mini_board/.gitignore

@@ -0,0 +1,2 @@
+_autosave-*
+*-backups

hardware/mini_board/picodvi.kicad_prl

@@ -1,6 +1,6 @@
 {
   "board": {
-    "active_layer": 44,
+    "active_layer": 31,
     "active_layer_preset": "",
     "auto_track_width": true,
     "hidden_nets": [],

hardware/mini_board/picodvi.kicad_pro

@@ -48,7 +48,13 @@
           "min_clearance": 0.15
         }
       },
-      "diff_pair_dimensions": [],
+      "diff_pair_dimensions": [
+        {
+          "gap": 0.0,
+          "via_gap": 0.0,
+          "width": 0.0
+        }
+      ],
       "drc_exclusions": [],
       "meta": {
         "filename": "board_design_settings.json",