zephyr: hello-world: add cmake_minimum_required
The sample fails to build without it now due to changes in the
boilerplate file.
# Top-level CMakeLists.txt for the skeleton application.
#
# Copyright (c) 2017 Open Source Foundries Limited
+# Copyright (c) 2018 Foundries.io Ltd
#
# SPDX-License-Identifier: Apache-2.0
#
# mcuboot, which is easy to customize on a per-board basis. It can be
# used as a starting point for new applications.
+cmake_minimum_required(VERSION 3.8)
+
# The default top-level application configuration is prj.conf.
# You can place additional board-specific files in boards/${BOARD}.conf,
# and they will be merged into the configuration along with prj.conf.
