srs_{ue,enb}: Add modifier to pass arbitrary cmd line arguments

Change-Id: Ie438c8aa9992dbc61b2133473a83d565cf5c7056
diff --git a/example/scenarios/mod-modem-args@.conf b/example/scenarios/mod-modem-args@.conf
new file mode 100644
index 0000000..dde7796
--- /dev/null
+++ b/example/scenarios/mod-modem-args@.conf
@@ -0,0 +1,3 @@
+modifiers:
+  modem:
+  - additional_args: ${param1}