nimble/ll: Allow to tune time needed for scheduling next conn event

This works by allowing to shorten connection event end time by specified
number of us. This value should be tuned only after measuring performance
as depends on various factors like build optimisation, cache, clock
speed etc.

nimble/controller/src/ble_ll_conn.c

@@ -939,6 +939,8 @@ ble_ll_conn_get_next_sched_time(struct ble_ll_conn_sm *connsm)
     rem_us = connsm->anchor_point_usecs;
     ble_ll_tmr_add_u(&ce_end, &rem_us, connsm->conn_itvl_usecs);
 
+    ce_end -= ble_ll_tmr_u2t_up(MYNEWT_VAL(BLE_LL_CONN_EVENT_END_MARGIN));
+
     if (ble_ll_sched_next_time(&next_sched_time)) {
         if (LL_TMR_LT(next_sched_time, ce_end)) {
             ce_end = next_sched_time;

nimble/controller/syscfg.yml

@@ -537,6 +537,15 @@ syscfg.defs:
         range: 1..257
         value: 32
 
+    BLE_LL_CONN_EVENT_END_MARGIN:
+        description: >
+            Extra time needed for scheduling next connection event. Setting this
+            value results in ending connection event sooner (in microseconds)
+            which gives more time to schedule next event. This value should be
+            tuned only after measuring performance as depends on various factors
+            like  build optimisation, cache, clock speed etc.
+        value: 0
+
     BLE_LL_STACK_SIZE:
         description: >
               This is the stack size for LL task.