atmega timer: Interrupt Pin

 A debug pin can be used to probe timer interrupts with an oscilloscope or
 other time measurement equipment. Thus, determine when an interrupt occurs
 and how long the timer ISR takes.
 The pin should be defined in the makefile as follows:

CFLAGS += -DDEBUG_TIMER_PORT=PORTF -DDEBUG_TIMER_DDR=DDRF \
            -DDEBUG_TIMER_PIN=PORTF4