fwu: make changes to access version agnostic structure fields

fwu: make changes to access version agnostic structure fields

With addition of support for version 2 of the FWU metadata structure,
the metadata information is collected into a version agnostic
structure. Make changes to the FWU functions so that the information
that was earlier obtained by reading the metadata structure is now
obtained through this version agnostic structure.

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