ms_srs: add config scheme to select NAS log level

we've provided only all_log_level so far but sometimes it's needed
to select the level per layer. This patch adds the ability
to do so for the NAS layer in the UE.

Change-Id: Iab2bce65e8af81f6d344849c97952e6441cb2846
diff --git a/sysmocom/defaults.conf b/sysmocom/defaults.conf
index d66e030..7939d3e 100644
--- a/sysmocom/defaults.conf
+++ b/sysmocom/defaults.conf
@@ -181,6 +181,7 @@
   rx_gain: 40
   freq_offset: 0
   log_all_level: warning
+  log_nas_level: warning
 
 amarisoftue:
   tx_gain: 89