fwu: metadata: add support for version 2 of the structure

fwu: metadata: add support for version 2 of the structure

Add support for version 2 of the FWU metadata structure. The top level
structure is kept separate through a config symbol. Most of the
fields, primarily used for providing information on updatable images
are common across the two versions.

Also change a few existing structure members used for image
identification to reflect the fact that these are GUIDs, and not
UUIDs.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Tested-by: Michal Simek <michal.simek@amd.com>