It may be convenient to use [pxe/redfish/...]kernel_append_params as
the universal defaults, while using {driver,instance}_info to only
append values. This change allows that by replacing %default% with
the value of the applicable configuration option.
An example use case: CoreOS requires an additional artifact (root
filesystem URL) when PXE booting.
While here, fix the PXE/iPXE interface documentation.
Change-Id: I829291ab5cc19ec2ca43bc45815d012697f0b408
5feb39844a