atch-atmega uart-mux-yun: __pipe_input_handle, don not alloc buffers

bathos_alloc_buffer seems slow; we do not use it anymore in critical
__pipe_input_handle function. In order not to use too much RAM, I here
reduced RAM reserved for allocator.

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>
main: call handle_events again till some pending_event is there

This improves performances when one event triggers another event: happens
in arch-atmega/uart-mux-yun.c, for instance.

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