test: Add a function to restart U-Boot

Add a proper function for this rather than using internal functions. Use it
in the single call site.

Also, do a restart at the end of the vboot test to reset to the normal
device tree.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Stephen Warren <swarren@nvidia.com>

test/py/u_boot_console_base.py

@@ -393,6 +393,11 @@ class ConsoleBase(object):
             pass
         self.p = None
 
+    def restart_uboot(self):
+        """Shut down and restart U-Boot."""
+        self.cleanup_spawn()
+        self.ensure_spawned()
+
     def get_spawn_output(self):
         """Return the start-up output from U-Boot