arch-atmega/spi.c: handle char 0 as 'end of enquiry' when polling is incoming

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>
tasks/mcuio_gpio_func.c: use GPIO_EVT_ defines

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>
gpio: implemented level irqs

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>
mcuio irq controller: do not automatically mask irqs

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