replace AppTimer on Nordic's nRF5x with a driver for RTC1.

AppTimer was an overhead; and it was also reseting the RTC from time to time.

Author: Rohit Grover

libraries/mbed/common/us_ticker_api.c

@@ -77,15 +77,17 @@ void us_ticker_insert_event(ticker_event_t *obj, timestamp_t timestamp, uint32_t
         prev = p;
         p = p->next;
     }
+
+    /* if we're at the end p will be NULL, which is correct */
+    obj->next = p;
+
     /* if prev is NULL we're at the head */
     if (prev == NULL) {
         head = obj;
         us_ticker_set_interrupt(timestamp);
     } else {
         prev->next = obj;
     }
-    /* if we're at the end p will be NULL, which is correct */
-    obj->next = p;
 
     __enable_irq();
 }