Fix DebugPort Implementation

Fix DebugPort Implementation

If the instance is managed as a global variable, the process tries to
access to the destroyed DebugPort instance while calling the exit handlers.

Change-Id: Ia7cfa9d241266921221ecf28ce8b4b22dc4b95d6
Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>