[example] Add SAMD21 Xplained Pro usb serial example

examples/samd21_xplained_pro/usbserial/main.cpp

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+/*
+ * Copyright (c) 2016-2017, Niklas Hauser
+ * Copyright (c) 2022, Christopher Durand
+ *
+ * This file is part of the modm project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+// ----------------------------------------------------------------------------
+
+#include <modm/board.hpp>
+#include <modm/processing/timer.hpp>
+
+using namespace Board;
+
+modm::IODeviceWrapper<UsbUart0, modm::IOBuffer::BlockIfFull> usb_io_device;
+modm::IOStream usb_stream(usb_io_device);
+
+int
+main()
+{
+	Board::initialize();
+	Board::initializeUsbFs();
+
+	tusb_init();
+
+	usb_stream << "Hello from USB" << modm::endl;
+
+	uint32_t counter(0);
+
+	modm::PeriodicTimer timer{500ms};
+	while (true)
+	{
+		tud_task();
+		if (timer.execute()) {
+			Led0::toggle();
+			usb_stream << "loop: " << counter++ << modm::endl;
+		}
+	}
+
+	return 0;
+}

examples/samd21_xplained_pro/usbserial/project.xml

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+<library>
+  <extends>modm:samd21-xplained-pro</extends>
+  <options>
+    <option name="modm:tinyusb:config">device.cdc</option>
+    <option name="modm:build:build.path">../../../build/samd21_xplained_pro/usbserial</option>
+  </options>
+  <modules>
+    <module>modm:tinyusb</module>
+    <module>modm:build:scons</module>
+    <module>modm:processing:timer</module>
+  </modules>
+</library>