Bluetooth: services: battery service enhancements

This commit moves the BLE GATT Battery service
from /samples/bluetooth/gatt to /subsys/bluetooth/services and
adds a Kconfig entry to enable and configure the service;
when enabled, it will register itself automatically.

Signed-off-by: Emanuele Di Santo <emdi@nordicsemi.no>
Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>

include/bluetooth/services/bas.h

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2018 Nordic Semiconductor ASA
+ * Copyright (c) 2016 Intel Corporation
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef ZEPHYR_INCLUDE_BLUETOOTH_SERVICES_BAS_H_
+#define ZEPHYR_INCLUDE_BLUETOOTH_SERVICES_BAS_H_
+
+/**
+ * @brief Battery Service (BAS)
+ * @defgroup bt_gatt_bas Battery Service (BAS)
+ * @ingroup bluetooth
+ * @{
+ *
+ * [Experimental] Users should note that the APIs can change
+ * as a part of ongoing development.
+ */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <zephyr/types.h>
+
+/** @brief Read battery level value.
+ *
+ * Read the characteristic value of the battery level
+ *
+ *  @return The battery level in percent.
+ */
+u8_t bt_gatt_bas_get_battery_level(void);
+
+/** @brief Update battery level value.
+ *
+ * Update the characteristic value of the battery level
+ * This will send a GATT notification to all current subscribers.
+ *
+ *  @param level The battery level in percent.
+ *
+ *  @return Zero in case of success and error code in case of error.
+ */
+int bt_gatt_bas_set_battery_level(u8_t level);
+
+
+#ifdef __cplusplus
+}
+#endif
+
+/**
+ * @}
+ */
+
+#endif /* ZEPHYR_INCLUDE_BLUETOOTH_SERVICES_BAS_H_ */