autopilot: add '--autopilot' run option

The '--autopilot' option makes it possible for a run script to detect if
it is being executed by the autopilot by

  if { [get_cmd_switch --autopilot] } { ... }

In that case, the run script could do an automated test, whereas the
test could be interactive when run without this option.

Fixes #2076

tool/autopilot

@@ -309,7 +309,7 @@ foreach platform $platforms {
 	if {[info exists ::env(RUN_OPT_AUTOPILOT)]} {
 		set kernel [exec echo $platform |& sed {s/_.*//}]
 
-		exec echo "RUN_OPT=--include boot_dir/$kernel $::env(RUN_OPT_AUTOPILOT)" >> $build_conf
+		exec echo "RUN_OPT=--include boot_dir/$kernel --autopilot $::env(RUN_OPT_AUTOPILOT)" >> $build_conf
 	}
 }