Configtool: write RX/TX_ENABLE_PIN as-is.

Because these two pins have no presence in Configtool's GUI, they
got dropped when writing a board config file. These pins are
needed for Gen3 Extruder Board support.

This should solve issue #179.

configtool/board.py

@@ -448,7 +448,11 @@ class Board:
             fp.write("//")
           fp.write(defineValueFormat % (t[0], v[0]))
         else:
-          print "Value key " + t[0] + " not found in GUI."
+          if t[0] == 'RX_ENABLE_PIN' or t[0] == 'TX_ENABLE_PIN':
+            # Known to be absent in the GUI, also won't be added anytime soon.
+            fp.write(ln)
+          else:
+            print("Value key " + t[0] + " not found in GUI.")
 
         continue