lib/event.c: handle_events, pending_events instead of direct CIRC_CNT call

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>
Kconfigs: configuration of PWM and ADC moved to tasks (was in arch-atmega)

PWM and ADC functions are now configured the same way as GPIOs.

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>
adc: rewritten in a more compact way, and ready to be used by iio driver on MPU

Note: some code has been removed since it was thought for asynchronous stream
of data, which is not implemented yet. It will be added again together
with linux driver support.

This permits some hundreds of byte saved in flash.

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>
trivial tasks mcuio_pwm_func: new FIXME comment

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>