button-handler:  Add button-handler service file

This service will start the button-handler app on the way to
standby after the mapper and the buttons app.

Tested:  Built an image and saw it start.

Change-Id: I4d08fa64f2a15486a4ccd33eaf7c6a89a281594e
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
2 files changed