samples: Add a sample for runtime chosen image

# Runtime chosen image sample application

This sample demonstrates how to use a non flash storage to retrieve the image

being booted. It was tested on a FRDM K64F. Both slots are used to store two

different images. The image to be booted is selected based on a button press.

## Build

Build mcuboot. First, ensure ZEPHYR_SDK_INSTALL_DIR is defined. From the

sample directory, run the following commands:

```

  source <path-to-zephyr>/zephyr-env.sh

  west build -p -b frdm_k64f ../../../boot/zephyr/ -- \

      -DEXTRA_ZEPHYR_MODULES=$PWD/hooks -DEXTRA_CONF_FILE="$PWD/sample.conf"

  west build -t flash

```

Then, build the sample application to be loaded. We need to build it twice, one

for each slot. From the sample

app directory (mcuboot/samples/non-flash-source/zephyr/app), run:

```

  west build -p -b frdm_k64f .

  west flash

```

Then change the overlay file to use the second slot. For instance, open

`boards/frdm_k64f.overlay` and change the line:

```

  zephyr,code-partition = &slot0_partition;

```

to:

```

    zephyr,code-partition = &slot1_partition;

```

And build and flash again:

```

  west build -b frdm_k64f .

  west flash

```

## Run

Open a serial terminal to see the output and reset the board. It shall boot the

image on slot0 by default. By keeping the SW2 button pressed during reset, the

bootloader will boot the one on slot1.

# SPDX-License-Identifier: Apache-2.0

cmake_minimum_required(VERSION 3.20.0)

find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})

project(non_flash_backend_app)

if(NOT DEFINED FROM_WHO)

  set(FROM_WHO Zephyr)

endif()

target_compile_definitions(app PRIVATE "-DMCUBOOT_HELLO_WORLD_FROM=\"${FROM_WHO}\"")

target_sources(app PRIVATE src/main.c)

/ {

	chosen {

		zephyr,code-partition = &slot0_partition;

	};

};

CONFIG_BOOTLOADER_MCUBOOT=y

CONFIG_MCUBOOT_SIGNATURE_KEY_FILE="./bootloader/mcuboot/root-rsa-2048.pem"

/*

 * Copyright (c) 2024 Intel Corporation

 *

 * SPDX-License-Identifier: Apache-2.0

 */

#include <zephyr/kernel.h>

#include <zephyr/sys/printk.h>

int main(void)

{

	printk("Hello World from %s on %s, slot %s!\n",

		   MCUBOOT_HELLO_WORLD_FROM, CONFIG_BOARD,

		   DT_PROP(DT_CHOSEN(zephyr_code_partition), label));

}