sencha: tell mpact cli to wait for a connection at startup

The mpact cli support assumes the target cpu has it's firmware loaded,
but this is done by the security core. Tell the simulator to wait for
a network connection before doing anything to properly synchronize the
cli & renode. Note this will block renode until there is a connection
so the cli should not be enabled unless it's going to be used.

NB: this requires an updated cheriot toolchain

Change-Id: Iae69b4ab57697de00a7b55c78ea7b5d802a05116