[Keyboard] Refactor handwired/splittest to support multiple boards (#6373)

* Refactor splittest to support multiple dev boards

* Refactor splittest to support multiple dev boards - revert change to number of RGB led

* Refactor splittest to support multiple dev boards - update docs

* Refactor splittest to support multiple dev boards - correct docs

* Refactor splittest to support multiple dev boards - update teensy master logic

keyboards/handwired/splittest/promicro/config.h

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+/* Copyright 2019
+ *
+ * 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/>.
+ */
+
+#pragma once
+
+#include "config_common.h"
+
+// wiring of each half
+#define MATRIX_ROW_PINS { B3 }
+#define MATRIX_COL_PINS { B6 }
+#define SPLIT_HAND_PIN F6
+#define SOFT_SERIAL_PIN D1
+
+/* ws2812 RGB LED */
+#define RGB_DI_PIN D3

keyboards/handwired/splittest/promicro/readme.md

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+# Pro Micro splittest
+
+To trigger keypress, short together pins *B3* and *B6* (marked on the PCB as *14* and *10*).
+
+## Wiring
+- Add switches to both Pro Micros across B3 and B6 pins
+- Add pull-up resistor to left side between VCC and F6
+- Add pull-down resistors to right side between GND and F6
+- Connect the following pins on both sides together: D0, D1, GND, VCC
+- Add I2C 4.7kOhm resistors between D0 and VCC, and D1 and VCC
+- Wire Di of RGB strip for each half to D3

keyboards/handwired/splittest/promicro/rules.mk

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+# 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 selection
+#   Teensy       halfkay
+#   Pro Micro    caterina
+#   Atmel DFU    atmel-dfu
+#   LUFA DFU     lufa-dfu
+#   QMK DFU      qmk-dfu
+#   atmega32a    bootloadHID
+BOOTLOADER = caterina
+
+
+# If you don't know the bootloader type, then you can specify the
+# Boot Section Size in *bytes* by uncommenting out the OPT_DEFS line
+#   Teensy halfKay      512
+#   Teensy++ halfKay    1024
+#   Atmel DFU loader    4096
+#   LUFA bootloader     4096
+#   USBaspLoader        2048
+# OPT_DEFS += -DBOOTLOADER_SIZE=4096