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/boards/shields/lily58/lily58_left.overlay

@@ -17,3 +17,6 @@
 		;
 };
 
+&bt_unpair_combo {
+	key-positions = <0 42>;
+};