Add Arduino compatibility layer

We can now directly compile arduino sketches (.pde) files.
Arduino compatible analogWrite works now.
But there is still a long way to go, serial I/O and timer stuff (delay,
millis etc) currently don't work (not tested but I don't expect this to
work).
It can be used in an arduino sketch or in a normal contiki program.
We get a PWM frequency of 490.2 Hz (a period of 2.040 ms), that's
Arduino compatible. If you need different frequencies see native timer
usage in examples/osd/pwm-example
In a contiki program you have to call arduino_pwm_timer_init to
initialize the timer before pwm works. The arduino sketch wrapper
already does this.
For running a sketch, see examples/osd/arduino-sketch

platform/osd-merkur/dev/hw-arduino.h

@@ -0,0 +1,73 @@
+/*
+ * Copyright (c) 2014, Ralf Schlatterbeck Open Source Consulting
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the Institute nor the names of its contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE INSTITUTE AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE INSTITUTE OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * This file is part of the Contiki operating system.
+ */
+
+/**
+ * \defgroup compatibility Arduino - Contiki
+ *
+ * This defines contiki-compatible hardware definitions for running
+ * arduino sketches (or just to call arduino-compatible function). 
+ * For now only for osd hardware, a similar file should exist for each
+ * arduino-compatible hardware.
+ *
+ * @{
+ */
+
+/**
+ * \file
+ *        Header file for arduino compatibility
+ * \author
+ *        Ralf Schlatterbeck <rsc@runtux.com>
+ *
+ */
+
+/*
+ * The OSD hardware only supports timer 3 for PWM, timer 2 is used by
+ * contiki for sleep/wakeup timing and is not usable for PWM.
+ */
+#define digitalPinToTimer(pin)                \
+    ( (pin) == 3                              \
+    ? TIMER3A                                 \
+    : ( (pin) == 4                            \
+      ? TIMER3B                               \
+      : ((pin == 5) ? TIMER3C : NOT_ON_TIMER) \
+      )                                       \
+    )
+
+/* Only init timer 3 with phase correct pwm 8-bit and prescaler 64 */
+#define arduino_pwm_timer_init() \
+  (hwtimer_ini (3, HWT_WGM_PWM_PHASE_8_BIT, HWT_CLOCK_PRESCALER_64, 0))
+
+/*
+ * VI settings, see coding style
+ * ex:ts=8:et:sw=2
+ */
+
+/** @} */