enb_srs: expose DL AWGN channel modifier for srsENB

we've added the modifier to configure Amarisoft eNB channel
model. This patch enables it for srsENB.

Change-Id: I30e65d0431b2d2792986128287caf8b23a22b2c1

src/osmo_gsm_tester/obj/enb_srs.py

@@ -194,6 +194,8 @@ class srsENB(enb.eNodeB, srslte_common):
         self.enable_pcap = util.str2bool(values['enb'].get('enable_pcap', 'false'))
         config.overlay(values, dict(enb={'enable_pcap': self.enable_pcap}))
 
+        config.overlay(values, dict(enb={'enable_dl_awgn': util.str2bool(values['enb'].get('enable_dl_awgn', 'false'))}))
+
         self._additional_args = []
         for add_args in values['enb'].get('additional_args', []):
             self._additional_args += add_args.split()

src/osmo_gsm_tester/templates/srsenb.conf.tmpl

@@ -193,8 +193,16 @@ enable = false
 # hst.fd_hz:         Doppler frequency in Hz
 # hst.init_time_s:   Initial time in seconds
 #####################################################################
+% if enb.enable_dl_awgn:
 [channel.dl]
-#enable        = false
+enable        = true
+% endif
+
+% if enb.enable_dl_awgn:
+[channel.dl.awgn]
+enable        = true
+snr           = ${enb.dl_awgn_snr}
+% endif
 
 [channel.dl.fading]
 #enable        = false