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/srsenb.conf.tmpl b/src/osmo_gsm_tester/templates/srsenb.conf.tmpl
index 81753d6..ac94313 100644
--- a/src/osmo_gsm_tester/templates/srsenb.conf.tmpl
+++ b/src/osmo_gsm_tester/templates/srsenb.conf.tmpl
@@ -272,7 +272,7 @@
 #pregenerate_signals  = false
 #tx_amplitude         = 0.6
 #link_failure_nof_err = 50
-rrc_inactivity_timer = 5000
+rrc_inactivity_timer = ${enb.inactivity_timer}
 #max_prach_offset_us  = 30
 #eea_pref_list = EEA0, EEA2, EEA1
 #eia_pref_list = EIA2, EIA1, EIA0