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%. 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;

unl0kr/main.c

@@ -591,7 +591,8 @@ int main(int argc, char *argv[]) {
     uint32_t timeout = conf_opts.general.timeout * 1000; /* ms */
     while(1) {
         if (!timeout || lv_disp_get_inactive_time(NULL) < timeout) {
-            lv_timer_periodic_handler();
+            uint32_t time_till_next = lv_timer_handler();
+            usleep(time_till_next * 1000);
         } else if (timeout) {
             shutdown();
         }