For the ATmega platform the ADC lines are not defined according to the GPIOs. Instead, they are defined from 0 to ADC_NUMOF-1, which in turn represents the bit in the corresponding ADC registers. The mapping from the line number to the corresponding ADC register/bit combination is done implicitly by the periph/adc implementation.
This adds a LED_PANIC macro which defines which LED,
or combination of LEDs should notify a panic error.
This is currently used to signal BADISR_vect errors.
