RGB underglow status support

Adds Glove80's status indicator using RGB underglow support. Requires ZMK
PR#999 and PR#1243.

The underglow status is able to show layer state, battery levels,
caps/num/scroll-lock, BLE and USB state. The underglow positions selected for
each of these indicators is configured using the new devicetree node
zmk,underglow-indicators, which takes an array of integer LED positions for each
feature.

app/dts/bindings/zmk,underglow-indicators.yaml

@@ -0,0 +1,35 @@
+# Copyright (c) 2020, The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: Underglow indicators
+
+compatible: "zmk,underglow-indicators"
+
+properties:
+  bat-lhs:
+    type: array
+    required: true
+  bat-rhs:
+    type: array
+    required: true
+  capslock:
+    type: int
+    required: true
+  numlock:
+    type: int
+    required: true
+  scrolllock:
+    type: int
+    required: true
+  layer-state:
+    type: array
+    required: true
+  ble-state:
+    type: array
+    required: true
+  usb-state:
+    type: int
+    required: true
+  output-fallback:
+    type: int
+    required: true