Keyboard: Port of TMK XT USB Converter (#3619)

* Converter: add XT USB converter from TMK

* Converter: port TMK code for USB converter to QMK

* Apply requested changes after code review by @drashna

- use "pragma once" instead of include guards
- use LAYOUT instead of KEYMAP
- allow customisation with matrix_init_user() and matrix_scan_user()
- set BOOTLOADER instead of BOOTLOADER_SIZE
- Add XT_ENABLE to OPT_DEFS

* Remove unnecessary lines

keyboards/converter/xt_usb/led.c

@@ -0,0 +1,22 @@
+/*
+Copyright 2016 Ethan Apodaca <papodaca@gmail.com>
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include "led.h"
+
+void led_set(uint8_t usb_led) {
+  //XT Keyboards do not have LEDs, nothing to do.
+}