Make POST device configurable.

Change-Id: If92b50ab3888518228d2d3b76f5c50c4aef968dd
Signed-off-by: Idwer Vollering <vidwer@gmail.com>
Reviewed-on: http://review.coreboot.org/4561
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>