enb: add basic TDD config option

this patch adds the basic notion of FDD and TDD duplexing modes
to the eNB object. So far we've always assume FDD.

Since only Amarisoft eNB supports TDD, the required config
template changes, etc. are only applied there.

The patch also adds a scenario to enable the default TDD config.

Change-Id: I37216b5bfdf527d221913283b6c41d3c8fd6b500
diff --git a/sysmocom/defaults.conf b/sysmocom/defaults.conf
index 419c3a4..8201d05 100644
--- a/sysmocom/defaults.conf
+++ b/sysmocom/defaults.conf
@@ -110,6 +110,7 @@
   id: 0x19B
   mcc: 901
   mnc: 70
+  duplex: fdd
   transmission_mode: 1
   num_cells: 1
   inactivity_timer: 20000