Add Levinson keyboard (#1723)

* Fork lets_split to levinson

Update subproject default

* Update Readme

* Pass LED backlight info from master to slave over serial

keyboards/levinson/split_util.h

@@ -0,0 +1,20 @@
+#ifndef SPLIT_KEYBOARD_UTIL_H
+#define SPLIT_KEYBOARD_UTIL_H
+
+#include <stdbool.h>
+#include "eeconfig.h"
+
+#define SLAVE_I2C_ADDRESS           0x32
+
+extern volatile bool isLeftHand;
+
+// slave version of matix scan, defined in matrix.c
+void matrix_slave_scan(void);
+
+void split_keyboard_setup(void);
+bool has_usb(void);
+void keyboard_slave_loop(void);
+
+void matrix_master_OLED_init (void);
+
+#endif