Initial USB HID implementation

- One button reported as gamepad button
- One LED controlable via application specific HID report and FlyWithLua

FlyWithLua/rpi2040_hid.lua

@@ -0,0 +1,20 @@
+device = hid_open(0x1209, 1)
+
+if device == nil then
+   print("No device!")
+else
+   dataref("low_vac", "sim/cockpit2/annunciators/low_vacuum", "readable")
+   local prev_low_vac
+   function low_vac_indicator()
+      if low_vac ~= prev_low_vac then
+	 if low_vac > 0 then
+	    hid_write(device, 2, 255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0)
+	 else
+	    hid_write(device, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0)
+	 end
+	 prev_low_vac = low_vac
+      end
+   end
+
+   do_every_frame("low_vac_indicator()")
+end