Silkscreen fixes/small BOM adjustments

- Update silk screen reference designators
- Update backside silk screen for Yellow revision 1.1
- Set U21 (SOIC-8 RTC) to assemble and U25 (MSOP-8) as DNP
- Update heatsink footprint to add heatsink outline to silk screen (#44)
- Mark orientation of D26, D27, D28 and D29 (fixes #39)
- Use pin-header footprint for PoE selector J13, it is now JP5

Signed-off-by: Stefan Agner <stefan@agner.ch>

Yellow.kicad_prl

@@ -1,6 +1,6 @@
 {
   "board": {
-    "active_layer": 0,
+    "active_layer": 37,
     "active_layer_preset": "",
     "auto_track_width": true,
     "hidden_nets": [],
@@ -18,7 +18,7 @@
       "footprints": true,
       "graphics": true,
       "keepouts": true,
-      "lockedItems": false,
+      "lockedItems": true,
       "otherItems": true,
       "pads": true,
       "text": true,