sencha: default cli port to -1 (no connection)

This enables wrappers to enable the cli without modifying the renode
configuration.

Change-Id: I34ec39c2cf1ede25bbd3b28e4627c537e85aa192

sencha.resc

diff --git a/sencha.resc b/sencha.resc
index ab84cc2..ce9b76f 100644
--- a/sencha.resc
+++ b/sencha.resc
@@ -60,9 +60,9 @@
 $cheriotLibrary ?= @out/cheriot/sim/librenode_mpact_cheriot.so
 sysbus.cpu1 CpuLibraryPath $cheriotLibrary
 
-# Enable the command line interface on port 4567
-$cli_port?= 4567
-#sysbus.cpu1 CliPort $cli_port
+# Override cli_port to enable the command line interface on the specified port.
+$cli_port?= -1
+sysbus.cpu1 CliPort $cli_port
 
 # Setup ram_smc to coordinate with cpu1
 sysbus.ram_smc CpuLibraryPath $cheriotLibrary