Add layer_state_set_kb hook

tmk_core/common/action_layer.c

@@ -57,8 +57,14 @@ void default_layer_xor(uint32_t state)
  */
 uint32_t layer_state = 0;
 
+__attribute__((weak))
+uint32_t layer_state_set_kb(uint32_t state) {
+    return state;
+}
+
 static void layer_state_set(uint32_t state)
 {
+    state = layer_state_set_kb(state);
     dprint("layer_state: ");
     layer_debug(); dprint(" to ");
     layer_state = state;