Add REBOOT_SCRIPT in TF agent environment (#381)

This patch allows export the reboot_script commands to a REBOOT_SCRIPT env variable
that can later be used within the TF agent script in order to reboot the DUT. That's
sometimes necessary when dealing with alternate provisioning methods (calling
'eval $REBOOT_SCRIPT').

device-connectors/src/testflinger_device_connectors/devices/__init__.py

@@ -198,6 +198,7 @@ def runtest(self, args):
 
         extra_env = {}
         extra_env["AGENT_NAME"] = config.get("agent_name", "")
+        extra_env["REBOOT_SCRIPT"] = ";".join(config.get("reboot_script", ""))
         if "env" not in config:
             config["env"] = {}
         config["env"].update(extra_env)