Simple async routines enabled for timer subsystem

[anacierdem]

Example usage:

#include <stdio.h>

#include <libdragon.h>

typedef struct
{
    async_state;
    timer_link_t *timer;
    int counter;
} task_state;

// This is still supported
void cb(int ovfl) {
    printf("ovfl %d\n", ovfl);
}

async task(void *pt)
{
    task_state *st = (task_state*) pt;
    async_begin(st);

    while(1) {
        st->timer = new_timer(TIMER_TICKS(1000000), TF_ONE_SHOT, cb);
        await(async_timer_expired(st->timer));
        printf("Count: %d\n", st->counter);
        st->counter++;
        delete_timer(st->timer);
    }

    async_end;
}

int main(void)
{
    console_init();
    timer_init();

    debug_init_usblog();
    console_set_debug(true);

    async_init();

    task_state pt = {};
    async_new(&pt);
    async_schedule(task, &pt);
}