underglow-layer: use devicetree & clean up code

app/include/zmk/rgb_underglow_layer.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+#include <zmk/keymap.h>
+
+
+#define ZMK_RGB_CHILD_LEN_PLUS_ONE(node) 1 +
+
+#define ZMK_RGBMAP_LAYERS_LEN                                                                      \
+    (DT_FOREACH_CHILD(DT_INST(0, zmk_underglow_layer), ZMK_RGB_CHILD_LEN_PLUS_ONE) 0)
+
+const int zmk_rgbmap_id(uint8_t layer);
+uint32_t *rgb_underglow_get_bindings(void);
+uint8_t rgb_underglow_top_layer(void);