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/rules.mk

@@ -0,0 +1,62 @@
+# MCU name
+MCU = atmega32u4
+
+# Processor frequency.
+#     This will define a symbol, F_CPU, in all source code files equal to the
+#     processor frequency in Hz. You can then use this symbol in your source code to
+#     calculate timings. Do NOT tack on a 'UL' at the end, this will be done
+#     automatically to create a 32-bit value in your source code.
+#
+#     This will be an integer division of F_USB below, as it is sourced by
+#     F_USB after it has run through any CPU prescalers. Note that this value
+#     does not *change* the processor frequency - it should merely be updated to
+#     reflect the processor speed set externally so that the code can use accurate
+#     software delays.
+F_CPU = 16000000
+
+
+#
+# LUFA specific
+#
+# Target architecture (see library "Board Types" documentation).
+ARCH = AVR8
+
+# Input clock frequency.
+#     This will define a symbol, F_USB, in all source code files equal to the
+#     input clock frequency (before any prescaling is performed) in Hz. This value may
+#     differ from F_CPU if prescaling is used on the latter, and is required as the
+#     raw input clock is fed directly to the PLL sections of the AVR for high speed
+#     clock generation for the USB and other AVR subsections. Do NOT tack on a 'UL'
+#     at the end, this will be done automatically to create a 32-bit value in your
+#     source code.
+#
+#     If no clock division is performed on the input clock inside the AVR (via the
+#     CPU clock adjust registers or the clock division fuses), this will be equal to F_CPU.
+F_USB = $(F_CPU)
+
+# Interrupt driven control endpoint task(+60)
+OPT_DEFS += -DINTERRUPT_CONTROL_ENDPOINT
+
+# Bootloader
+#     This indicates which bootloader is present on the board.
+# BOOTLOADER = caterina # Pro Micro
+BOOTLOADER = halfkay # Teensy
+
+
+# Build Options
+#   comment out to disable the options.
+#
+BOOTMAGIC_ENABLE = no	# Virtual DIP switch configuration(+1000)
+MOUSEKEY_ENABLE = yes	# Mouse keys(+4700)
+EXTRAKEY_ENABLE = yes	# Audio control and System control(+450)
+CONSOLE_ENABLE = yes	# Console for debug(+400)
+COMMAND_ENABLE = yes   # Commands for debug and configuration
+NKRO_ENABLE = yes	# USB Nkey Rollover
+XT_ENABLE = yes
+CUSTOM_MATRIX = yes
+
+
+# Optimize size but this may cause error "relocation truncated to fit"
+#EXTRALDFLAGS = -Wl,--relax
+
+SRC = matrix.c led.c