examples: added Skald Eddystone example

examples/skald_eddystone/Makefile

+# name of your application
+APPLICATION = skald_eddystone
+
+# If no BOARD is found in the environment, use this default:
+BOARD ?= nrf52dk
+
+# This has to be the absolute path to the RIOT base directory:
+RIOTBASE ?= $(CURDIR)/../..
+
+# include Skald
+USEMODULE += skald_eddystone
+
+# Comment this out to disable code in RIOT that does safety checking
+# which is not needed in a production environment but helps in the
+# development process:
+# CFLAGS += -DDEVELHELP
+
+# Change this to 0 show compiler invocation lines by default:
+QUIET ?= 1
+
+include $(RIOTBASE)/Makefile.include

examples/skald_eddystone/README.md

+# Skald iBeacon Example
+
+This example demonstrates the usage of `Skald` for creating an Google
+`Eddystone` beacon, advertising an`Eddystone-URI` and an `Eddystone-URL`
+concurrently.
+
+Simply compile and flash, and verify your newly created beacon with any type of
+BLE scanner.

examples/skald_eddystone/main.c

+/*
+ * Copyright (C) 2018 Freie Universität Berlin
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     examples
+ * @{
+ *
+ * @file
+ * @brief       BLE Eddystone beacon example using Skald
+ *
+ * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
+ *
+ * @}
+ */
+
+#include "log.h"
+
+#include "net/skald/eddystone.h"
+
+/* example of an Eddystone URI:
+ * - namespace (ASCII): 'supercool!'
+ * - instance  (ASCII): `_RIOT_` */
+#define URI_NAMESPACE   { 0x73, 0x75, 0x70, 0x65, 0x72, \
+                          0x63, 0x6f, 0x6f, 0x6c, 0x21 }
+#define URI_INSTANCE    { 0x5f, 0x52, 0x49, 0x4f, 0x54, 0x5f }
+
+/* advertise this short URL, points to https://www.riot-os.org */
+#define URL             "bit.ly/2Ep11dm"
+/* calibrated TX power value */
+#define TX_PWR          (0U)
+
+
+/* allocate two advertising contexts, one for Eddystone-URL and one for
+ * Eddystone-URI */
+static skald_ctx_t _ctx_uid;
+static skald_ctx_t _ctx_url;
+
+int main(void)
+{
+    LOG_INFO("Skald and the tail of Eddystone\n");
+
+    /* advertise the defined URI */
+    skald_eddystone_uid_t uid = { URI_NAMESPACE, URI_INSTANCE };
+    skald_eddystone_uid_adv(&_ctx_uid, &uid, TX_PWR);
+
+    /* also advertise the defined short-URL */
+    skald_eddystone_url_adv(&_ctx_url, EDDYSTONE_URL_HTTPS, URL, TX_PWR);
+
+    return 0;
+}