fwu: move meta-data management in core

Instead of each i/f having to implement their own meta-data verification
and storage, move the logic in common code. This simplifies the i/f code
much simpler and compact.

Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Sughosh Ganu <sughosh.ganu@linaro.org>