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mboxd needs to receive a HUP signal to reset it
in between boots, so this will do that on a power
off so it will be ready for the next boot.

This adds about 25s to the power off sequence.

Change-Id: Icaf86de0c1fda297e74229e5ab0b182dfba96456
Signed-off-by: Matt Spinler <spinler@us.ibm.com>