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;

app/src/ble_unpair_combo.c

@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2020 Peter Johanson
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <device.h>
+#include <init.h>
+
+#include <logging/log.h>
+
+#define DT_DRV_COMPAT zmk_bt_unpair_combo
+
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+#include <zmk/ble.h>
+#include <zmk/event-manager.h>
+#include <zmk/events/position-state-changed.h>
+
+
+static u8_t combo_state;
+
+const u32_t key_positions[] = DT_INST_PROP(0, key_positions);
+#define KP_LEN DT_INST_PROP_LEN(0, key_positions)
+
+int index_for_key_position(u32_t kp)
+{
+    for (int i = 0; i < KP_LEN; i++) {
+        if (key_positions[i] == kp) {
+            return i;
+        }
+    }
+
+    return -1;
+}
+
+int unpair_combo_listener(const struct zmk_event_header *eh)
+{
+    if (is_position_state_changed(eh)) {
+        const struct position_state_changed *psc = cast_position_state_changed(eh);
+
+        int kp_index = index_for_key_position(psc->position);
+        if (kp_index < 0) {
+            return 0;
+        }
+
+        WRITE_BIT(combo_state, kp_index, psc->state);
+    }
+
+    return 0;
+};
+
+void unpair_combo_work_handler(struct k_work *work)
+{
+    for (int i = 0; i < KP_LEN; i++) {
+        if (!(combo_state & BIT(i))) {
+            LOG_DBG("Key position %d not held, skipping unpair combo", key_positions[i]);
+            return;
+        }
+    }
+
+    zmk_ble_unpair_all();
+};
+
+struct k_delayed_work unpair_combo_work;
+
+int zmk_ble_unpair_combo_init(struct device *_unused)
+{
+    k_delayed_work_init(&unpair_combo_work, unpair_combo_work_handler);
+    k_delayed_work_submit(&unpair_combo_work, K_SECONDS(2));
+
+    return 0;
+};
+
+ZMK_LISTENER(zmk_ble_unpair_combo, unpair_combo_listener);
+ZMK_SUBSCRIPTION(zmk_ble_unpair_combo, position_state_changed);
+
+SYS_INIT(zmk_ble_unpair_combo_init,
+        APPLICATION,
+        CONFIG_APPLICATION_INIT_PRIORITY);