use OS sleep instead of polling to reduce CPU usage

without this lvgl never releases the CPU, and just pegs one of the cores to 100% (with this, idle CPU use is < 1%). see https://github.com/lvgl/lvgl/blob/master/docs/porting/timer_handler.rst

buffyboard/main.c

@@ -284,7 +284,8 @@ int main(int argc, char *argv[]) {
 
     /* Periodically run timer / task handler */
     while(1) {
-        lv_timer_periodic_handler();
+        uint32_t time_till_next = lv_timer_handler();
+        usleep(time_till_next * 1000);
     }
 
     return 0;