board: arm: google_kukui: Add a sample for kukui

It is useful to use kukui with a shell to test out its features., Add a
sample for this.

Signed-off-by: Simon Glass <sjg@chromium.org>

samples/boards/google_kukui/CMakeLists.txt

@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.13.1)
+
+find_package(Zephyr HINTS $ENV{ZEPHYR_BASE})
+project(google_kukui)
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

samples/boards/google_kukui/README.rst

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+.. _google_kukui:
+
+Kukui general features
+######################
+
+Overview
+********
+
+This provides access to :ref:`Kukui <google_kukui_board>` through a serial shell
+so you can try out the supported features, including I2C, GPIO and flash access.
+
+Building
+********
+
+The sample can be built as follows:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/boards/google_kukui
+   :board: google_kukui
+   :goals: build
+   :compact:
+
+Sample Output
+=============
+
+.. code-block:: console
+
+    Welcome to Google Kukui
+
+    uart:~$    (press tab)
+      clear    flash    gpio     help     history  i2c      resize   shell
+    uart:~$ i2c read I2C_2 36
+    00000000: 82 00 00 ff 80 7f 00 ff  00 00 c1 0a c8 5b 0c 62 |........ .....[.b|

samples/boards/google_kukui/prj.conf

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+# Enable all useful shell commands (i.e. those with driver support)
+CONFIG_SHELL=y
+CONFIG_SHELL_BACKENDS=y
+CONFIG_SHELL_BACKEND_SERIAL=y
+CONFIG_GPIO_SHELL=y
+CONFIG_I2C_SHELL=y

samples/boards/google_kukui/sample.yaml

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+sample:
+  name: Kukui general features
+tests:
+  sample.board.google_kukui:
+    platform_whitelist: google_kukui
+    tags: ec

samples/boards/google_kukui/src/main.c

@@ -0,0 +1,13 @@
+/*
+ * Copyright 2020 Google LLC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr.h>
+#include <sys/printk.h>
+
+void main(void)
+{
+	printk("Welcome to Google Kukui\n");
+}