enb: expose RRC inactivity timer

allow the user to specifiy the RRC inactiviy timer used in the eNB.
set the default value to 20s

Change-Id: I6a264b8dee893b883d40a48831cbb47eb15e1e7b
diff --git a/src/osmo_gsm_tester/templates/amarisoft_enb.cfg.tmpl b/src/osmo_gsm_tester/templates/amarisoft_enb.cfg.tmpl
index 155ad47..2893b9e 100644
--- a/src/osmo_gsm_tester/templates/amarisoft_enb.cfg.tmpl
+++ b/src/osmo_gsm_tester/templates/amarisoft_enb.cfg.tmpl
@@ -223,7 +223,7 @@
 
     /* (in ms) send RRC connection release after this time of network
        inactivity */
-    inactivity_timer: 10000,
+    inactivity_timer: ${enb.inactivity_timer},
 
     /* SRB configuration */
     srb_config: [