host/gap: add function that returns free advertising instance

ble_gap_adv_get_free_instance browses through list of advertising
instances and returns first that is not yet configured. Previously,
user had to track which instance is used by themselves, and could only
check if advertising is active, via ble_gap_ext_adv_active

nimble/host/include/host/ble_gap.h

@@ -1692,6 +1692,14 @@ int ble_gap_ext_adv_clear(void);
  *
  */
 int ble_gap_ext_adv_active(uint8_t instance);
+
+/**
+ * Returns first not configured advertising instance.
+ *
+ * @return advertising instance if one was found, negative error code otherwise
+ *
+ */
+int ble_gap_adv_get_free_instance(void);
 #endif
 
 /* Periodic Advertising */

nimble/host/src/ble_gap.c

@@ -1390,6 +1390,20 @@ int ble_gap_ext_adv_active(uint8_t instance)
     }
     return ble_gap_adv_active_instance(instance);
 }
+
+int
+ble_gap_adv_get_free_instance(void)
+{
+    int i;
+
+    for (i = 0; i < BLE_ADV_INSTANCES; i++) {
+        if (!ble_gap_slave[i].configured) {
+            return i;
+        }
+    }
+
+    return -BLE_HS_ENOENT;
+}
 #endif
 
 /**