style(fwu): change the metadata fields to align with specification

Change the names of some FWU metadata structure members to have them
align with the wording used in the corresponding specification. Use
the GUID type instead of UUID as the fields described in the
specification are GUIDs. Make corresponding changes to the code that
accesses these fields. No functional changes are introduced by the
patch.

Change-Id: Id3544ed1633811b0eeee2bf99477f9b7e6667044
Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>