trunner: armv7a9: add missing clear_buffer

Adds omitted clear_buffer to prevent caching output from previous step in to another one.

JIRA: CI-521

trunner/target/armv7a9.py

@@ -49,6 +49,7 @@ def _reboot_by_debugger(self):
                 "-c init; reset; continue",
             ],
         ).run()
+        self.dut.clear_buffer()
 
     def _reboot_soft(self):
         self._reboot_hard()