Add external extruder control board prototype.

EExtruder/BuckConverter.dcm

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+EESchema-DOCLIB  Version 2.0
+#
+#End Doc Library

EExtruder/BuckConverter.lib

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+EESchema-LIBRARY Version 2.4
+#encoding utf-8
+#
+# DCBuck
+#
+DEF DCBuck U 0 40 Y Y 1 F N
+F0 "U" -100 -500 50 H V C CNN
+F1 "DCBuck" 0 350 50 H V C CNN
+F2 "" 0 0 50 H I C CNN
+F3 "" 0 0 50 H I C CNN
+DRAW
+S -250 300 250 -450 0 1 0 N
+X Vin 1 -350 150 100 R 50 50 1 1 W
+X GND_In 2 -350 -300 100 R 50 50 1 1 W
+X Vout 3 350 150 100 L 50 50 1 1 w
+X GND_Out 4 350 -400 100 L 50 50 1 1 w
+ENDDRAW
+ENDDEF
+#
+#End Library