samples: zephyr: add hello-world

This is a minimal example application that works on various boards,
without target-specific definitions, device tree overlay files, etc.

It's been tested on frdm_k64f, 96b_carbon, and 96b_nitrogen.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>

samples/zephyr/hello-world/Makefile

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+# Top-level Makefile for the skeleton application.
+#
+# This provides a basic application structure suitable for loading by
+# mcuboot, which is easy to customize on a per-board basis. It can be
+# used as a starting point for new applications.
+
+# The default board is FRDM-K64F. This can be overridden at
+# the command line for other boards supported by Zephyr.
+BOARD ?= frdm_k64f
+# The top-level application configuration is prj.conf. This can also
+# be overridden at the command line if you add another one.
+CONF_FILE ?= prj.conf
+
+# This will merge any board-specific extras from boards/$(BOARD).conf
+# into the configuration.
+CONF_FILE += $(wildcard boards/$(BOARD).conf)
+# These additional "local" files, if present, will be merged in as
+# well, but they are ignored by Git.
+CONF_FILE += $(wildcard local.conf) $(wildcard boards/$(BOARD)-local.conf)
+
+# Zephyr uses Device Tree (DT) to describe some board hardware
+# configuration.
+#
+# For this simple example, all we need to tell DT is where on the chip
+# flash to link this application image so mcuboot can find it. We do
+# this with a device tree overlay file.
+#
+# See the Zephyr documentation for more information on DT:
+# https://www.zephyrproject.org/doc/dts/device_tree.html
+DTC_OVERLAY_FILE := $(CURDIR)/dts.overlay
+export DTC_OVERLAY_FILE
+# If you want to do your DT overlay on a per-board basis (say, if you
+# need to support multiple different boards, each of which needs
+# slightly different DT overlays), then comment the above lines and
+# uncomment the following ones. You can then add board-specific
+# overlay files named boards/$(BOARD).overlay.
+#
+# DTC_OVERLAY_DIR := $(CURDIR)/boards
+# export DTC_OVERLAY_DIR
+
+# This string ends up getting printed in the device console
+FROM_WHO ?= "Zephyr"
+CFLAGS += -DMCUBOOT_HELLO_WORLD_FROM=\"$(FROM_WHO)\"
+
+# The Zephyr Makefiles do the rest.
+include $(ZEPHYR_BASE)/Makefile.inc

samples/zephyr/hello-world/README.rst

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+This is a "Hello world" skeleton application which can be used as a
+starting point for Zephyr application development using mcuboot.
+
+It includes the configuration "glue" needed to make the application
+loadable by mcuboot in addition to a basic Zephyr hello world
+application's code.

samples/zephyr/hello-world/boards/.gitignore

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+*-local.conf

samples/zephyr/hello-world/boards/README.rst

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+You can place per-board configuration and device tree overlays
+here. See the comments in the Makefile for more information.

samples/zephyr/hello-world/dts.overlay

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+/*
+ * Basic Device Tree overlay file for chain-loading by mcuboot.
+ * See the Makefile for more information.
+ */
+
+/ {
+	chosen {
+		zephyr,code-partition = &slot0_partition;
+	};
+};

samples/zephyr/hello-world/prj.conf

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+# Print a banner on the UART on startup.
+CONFIG_BOOT_BANNER=y
+CONFIG_BUILD_TIMESTAMP=y
+
+# TEXT_SECTION_OFFSET is used to leave space in the binary for the
+# mcuboot header.
+#
+# This is a conservative value which should work for most boards, at
+# the cost of unnecessarily reducing the amount of usable flash a bit
+# on some boards.
+CONFIG_TEXT_SECTION_OFFSET=0x200

samples/zephyr/hello-world/sample.yaml

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+sample:
+  name: Application Skeleton
+  description: Basic "hello world" application, but loadable by mcuboot
+  platforms: all
+tests:
+    - test:
+        build_only: true
+        tags: samples tests
+        min_ram: 16

samples/zephyr/hello-world/src/Makefile

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+obj-y = main.o

samples/zephyr/hello-world/src/main.c

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+/*
+ * Copyright (c) 2017 Linaro, Ltd.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr.h>
+#include <misc/printk.h>
+
+void main(void)
+{
+	printk("Hello World from %s on %s!\n",
+	       MCUBOOT_HELLO_WORLD_FROM, CONFIG_BOARD);
+}