Add an overlay to emit a tarmac trace.

This is a generally useful overlay that could be used quite often to get
a Tarmac trace from the FVP, for debugging for example:

  $ shrinkwrap run --overlay tarmac-trace.yaml --rtvar TARMAC_TRACE_FILE=my_trace.tarmac

The TARMAC_TRACE_FILE has a default value (trace.tarmac) so the trace file
name can be left out of the shrinkwrap invocation.

Signed-off-by: Arnaud de Grandmaison <arnaud.degrandmaison@arm.com>