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/src/ble.c

@@ -190,6 +190,12 @@ static int zmk_ble_init(struct device *_arg)
     return 0;
 }
 
+int zmk_ble_unpair_all()
+{
+    LOG_DBG("");
+    return bt_unpair(BT_ID_DEFAULT, NULL);
+};
+
 bool zmk_ble_handle_key_user(struct zmk_key_event *key_event)
 {
     zmk_key key = key_event->key;