fwu: metadata: add a version agnostic structure

fwu: metadata: add a version agnostic structure

The FWU specification now has two versions of the FWU metadata
structure, and both are to be supported. Introduce a version agnostic
structure for storing information about the FWU updatable images. This
allows for a split of common version agnostic FWU code and version
specific code.

The version specific code is then responsible for arranging the data
as per the corresponding metadata structure before it gets written to
the metadata partitions.

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