boards: Add support for the Arduino Nano

The Arduino Nano board is the cheapest member of the Arduino family and used the
same MCU as the Arduino Uno. It differs in the form factor (the Nano is much
smaller), it uses an integrated FT232RL TTL adapter instead of an ATmega16u2 to
provide access to the serial console via USB, and it uses a different
bootloader (which occupies 2 KiB of the 32 KiB flash instead of 0.5 KiB occupied
on the Arduino Uno). This commit mostly copy pastes code from the Arduino Uno.

boards/arduino-nano/Makefile

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+MODULE = board
+
+DIRS = $(RIOTBOARD)/common/arduino-atmega
+
+include $(RIOTBASE)/Makefile.base

boards/arduino-nano/Makefile.dep

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+USEMODULE += boards_common_arduino-atmega
+
+include $(RIOTBOARD)/common/arduino-atmega/Makefile.dep

boards/arduino-nano/Makefile.features

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+include $(RIOTBOARD)/common/arduino-atmega/Makefile.features
+
+include $(RIOTCPU)/atmega328p/Makefile.features

boards/arduino-nano/Makefile.include

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+# define the cpu used by the Arduino Nano board
+export CPU = atmega328p
+
+# configure the terminal program
+PORT_LINUX  ?= /dev/ttyUSB0
+PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
+BAUD        ?= 9600
+
+PROGRAMMER ?= arduino
+
+ifeq (arduino,$(PROGRAMMER))
+  # the Arduino Nano bootloader is 2KiB in size
+  BOOTLOADER_SIZE ?= 2048
+  # the Nano's bootloader uses 57600 baud for programming
+  FFLAGS_EXTRA += -b 57600
+else
+  # not using the bootloader for programming, thus the whole flash can be used
+  BOOTLOADER_SIZE ?= 0
+endif
+
+ROM_RESERVED ?= $(BOOTLOADER_SIZE)
+
+include $(RIOTBOARD)/common/arduino-atmega/Makefile.include

boards/arduino-nano/doc.txt

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+/**
+@defgroup    boards_arduino-nano Arduino Nano
+@ingroup     boards
+@brief       Support for the Arduino Nano board
+
+## Overview
+
+The Arduino Nano is the cheapest member of the Arduino family. It is based on
+Atmel's AVR architecture and sports an ATmega328p MCU. It is like many Arduinos
+extensible by using shields.
+
+### MCU
+| MCU           | ATmega328p                                    |
+|:------------- |:--------------------------------------------- |
+| Family        | AVR/ATmega                                    |
+| Vendor        | Atmel                                         |
+| RAM           | 2 KiB                                         |
+| Flash         | 32 KiB (2 KiB reserved for the bootloader)    |
+| Frequency     | 16 MHz                                        |
+| Timers        | 3 (2x 8bit, 1x 16bit)                         |
+| ADCs          | 6 analog input pins                           |
+| UARTs         | 1                                             |
+| SPIs          | 1                                             |
+| I2Cs          | 1 (called TWI)                                |
+| Vcc           | 5.0V                                          |
+| MCU Datasheet | [ATmega328p datasheet](http://ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061A.pdf) |
+| Board Manual  | [Board Manual](https://www.arduino.cc/en/uploads/Main/ArduinoNanoManual23.pdf) |
+
+## Flashing the device
+Flashing RIOT on the Arduino Nano is quite straight forward, just connect your
+Arduino Nano via the USB connector to your host computer and type:
+
+`make BOARD=arduino-nano flash`
+
+This should take care of everything!
+
+We use the open `avrdude` tool to write the new code into the ATmega328p's
+flash
+
+##Caution
+Don't expect having a working network stack due to very limited resources.
+ */

boards/arduino-nano/include/board.h

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+/*
+ * Copyright (C) 2017   Freie Universität Berlin
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     boards_arduino-nano
+ * @{
+ *
+ * @file
+ * @brief       Board specific definitions for the Arduino Uno board
+ *
+ * @author      Martine Lenders <m.lenders@fu-berlin.de>
+ */
+
+#ifndef BOARD_H
+#define BOARD_H
+
+#include "board_common.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* BOARD_H */
+/** @} */