feeature(bt): Add "unpair combo" on startup.

* Especially for splits, we need the ability to
  unpair all paired devices as  sledgehammer
  if we need to "reset things", and doing so via
  keymaps isn't suitable.
* Allows shields to define a collection of key
  positions that if all held 2 seconds after
  startup, will unpair all existing pairs for the
  device.

app/dts/bindings/zmk,bt-unpair-combo.yaml

@@ -0,0 +1,12 @@
+# Copyright (c) 2020, Pete Johanson
+# SPDX-License-Identifier: MIT
+
+description: |
+  Defines a set of key positions that will unpair all BT devices if held on startup.
+
+compatible: "zmk,bt-unpair-combo"
+
+properties:
+  key-positions:
+    type: array
+    required: true