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/include/zmk/ble.h

@@ -3,5 +3,5 @@
 
 #include <zmk/keys.h>
 
-int zmk_ble_init();
+int zmk_ble_unpair_all();
 bool zmk_ble_handle_key_user(struct zmk_key_event *key_event);