things in bootloader (open) main.c to look at:
APP_TIMER_DEF()/app_timer_create vs app_timer_id_t/app_timer_start? (taken from context in dfu_progress_led_timeout_handler?)
APP_TIMER_TICKS(<some time ms>);
 led_sb_init_params_t led_sb_init_param = LED_SB_INIT_DEFAULT_PARAMS(BSP_LED_1_MASK);
led_softblink_init
led_softblink_start
led_softblink_off_time_set(ticks)
led_softblink_on_time_set(ticks)
led_softblink_stop
 NRF_LOG_INIT(app_timer_cnt_get);
https://trello.com/c/BkTCUrXy/19-timer-setup-for-leds