Refactor A/D conversion in adc.c

Now the necessary settings are in adc.h. Refactored to allow repeated
ADC reads without reinitialization. Arduino allows setting
analogReference, this is now also implemented.
ADC is now initialized to sane values in apps/arduino/arduino-process.c
dev/arduino/arduino-compat.h now has all hardware independent settings
for arduino (some moved from platform/osd-merkur/dev/hw-arduino.h).
turnOffPWM re-implemented with hw_timer, removed from wiring_digital.c
ADC-specific arduino stuff moved to arduino-compat.h
Arduinos wiring_analog no longer necessary.
arduino-sketch example now reads analog inputs 1 and 5 using analogRead.

apps/arduino/arduino-process.c

@@ -51,6 +51,7 @@
 
 #include "arduino-process.h"
 #include "hw_timer.h"
+#include "adc.h"
 #include "hw-arduino.h"
 
 PROCESS(arduino_sketch, "Arduino Sketch Wrapper");
@@ -60,6 +61,7 @@ PROCESS_THREAD(arduino_sketch, ev, data)
   PROCESS_BEGIN();
 
   arduino_pwm_timer_init ();
+  adc_init ();
   setup ();
   while (1) {
     loop ();