[settings] Explicitly separate the concept of a completed fetched setting

[settings] Explicitly separate the concept of a completed fetched setting

The fetch_setting() family of functions may currently modify the
definition of the specified setting (e.g. to add missing type
information).  Clean up this interface by requiring callers to provide
an explicit buffer to contain the completed definition of the fetched
setting, if required.

Signed-off-by: Michael Brown <mcb30@ipxe.org>