Add 'exit' hftest command

A device may need to exit out of the hftest environment, e.g. an Android
device would otherwise always reboot back to hftest and would not be
able to flash a new image.

Add a hftest_device_exit_test_environment() to the hftest_device driver,
where this can be implemented in a board-specific way.

The default PSCI implementation will not support this command and will
print an error message.

Change-Id: I60a92aac21dfbc8b6476f6efe694f05b0967a27d
diff --git a/test/hftest/standalone_main.c b/test/hftest/standalone_main.c
index 91b53b5..8f0cec9 100644
--- a/test/hftest/standalone_main.c
+++ b/test/hftest/standalone_main.c
@@ -60,6 +60,11 @@
 		goto out;
 	}
 
+	if (memiter_iseq(&command, "exit")) {
+		hftest_device_exit_test_environment();
+		goto out;
+	}
+
 	if (memiter_iseq(&command, "json")) {
 		hftest_json();
 		goto out;