Add references for is_keyboard_left() (#10850)

* Add references for is_keyboard_left()

* Remove proto from bootmagic_lite.c

tmk_core/common/bootmagic_lite.c

@@ -1,7 +1,5 @@
 #include "quantum.h"
 
-bool is_keyboard_left(void);
-
 /** \brief Reset eeprom
  *
  * ...just incase someone wants to only change the eeprom behaviour

tmk_core/common/keyboard.c

@@ -222,6 +222,12 @@ void keyboard_setup(void) {
  */
 __attribute__((weak)) bool is_keyboard_master(void) { return true; }
 
+/** \brief is_keyboard_left
+ *
+ * FIXME: needs doc
+ */
+__attribute__((weak)) bool is_keyboard_left(void) { return true; }
+
 /** \brief should_process_keypress
  *
  * Override this function if you have a condition where keypresses processing should change:

tmk_core/common/keyboard.h

@@ -63,6 +63,8 @@ void keyboard_task(void);
 void keyboard_set_leds(uint8_t leds);
 /* it runs whenever code has to behave differently on a slave */
 bool is_keyboard_master(void);
+/* it runs whenever code has to behave differently on left vs right split */
+bool is_keyboard_left(void);
 
 void keyboard_pre_init_kb(void);
 void keyboard_pre_init_user(void);