Enable usrp-tx-thread-config in UE

Setting this option will start a separate thread for writing samples to
USRP.

executables/nr-ue.c

@@ -802,6 +802,8 @@ void *UE_thread(void *arg)
   UE->is_synchronized = 0;
   int tmp2 = UE->rfdevice.trx_start_func(&UE->rfdevice);
   AssertFatal(tmp2 == 0, "Could not start the device\n");
+  if (usrp_tx_thread == 1)
+    UE->rfdevice.trx_write_init(&UE->rfdevice);
 
   notifiedFIFO_t nf;
   initNotifiedFIFO(&nf);