samples: Added serial_event sample

Add a sample to demonstrate how to use Serial APIs

Author: soburi

samples/serial_event/CMakeLists.txt

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+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+
+set(DTC_OVERLAY_FILE $ENV{ZEPHYR_BASE}/../modules/lib/Arduino-Zephyr-API/variants/${BOARD}/${BOARD}.overlay)
+
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(serial_event)
+
+target_sources(app PRIVATE src/app.cpp)
+
+zephyr_compile_options(-Wno-unused-variable -Wno-comment)

samples/serial_event/README.rst

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+.. _serial_event:
+
+Serial Event
+############
+
+Overview
+********
+
+The serial_event sample echo back serial input data.
+
+Building and Running
+********************
+
+Build and flash serial_event sample as follows,
+
+```sh
+$> west build -p -b arduino_nano_33_ble sample/serial_event/
+
+$> west flash --bossac=/home/$USER/.arduino15/packages/arduino/tools/bossac/1.9.1-arduino2/bossac
+```

samples/serial_event/prj.conf

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+CONFIG_ARDUINO_API=y

samples/serial_event/src/app.cpp

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+/*
+ * Copyright (c) 2022 TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <Arduino.h>
+
+void setup() {
+  Serial.begin(115200);
+}
+
+void loop() {
+}
+
+void serialEvent() {
+  while(Serial.available()) {
+    Serial.print((char)Serial.read());
+  }
+}