In file samd21_pins.c line 122, pin PB07 is not assigned to ADC_INPUT but to TOUCH:
PIN(PB07, EXTINT_CHANNEL(7), TOUCH(ADC_POSITIVE_INPUT_PIN15), TOUCH(13),

I think the correct way is:
PIN(PB07, EXTINT_CHANNEL(7), ADC_INPUT(ADC_POSITIVE_INPUT_PIN15), TOUCH(13),