fwu: metadata: add functions for handling version specific metadata fields

fwu: metadata: add functions for handling version specific metadata fields

Support is being added in U-Boot for version 2 of the FWU
metadata. Support for this version is to co-exist with version 1
support. To achieve this, a common, version agnostic structure has
been added to keep information provided by the FWU metadata
structure.

Add API's to handle the version specific FWU metadata fields. The
version agnostic structure gets initialized at boot by reading the FWU
metadata. Updates to the FWU metadata result in the API's getting
called to populate the version specific fields of the strucure, before
the metadata gets written to the storage media.

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