fwu: v2: try reading both copies of metadata

fwu: v2: try reading both copies of metadata

In the version 2 of the FWU metadata, the metadata is broken into two
parts, a top-level structure, which provides information on the total
size of the structure among other things. Try reading the primary
partition first, and if that fails, try reading the secondary
partition. This will help in the scenario where the primary metadata
partition has been corrupted, but the secondary partition is intact.

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