fwu: mtd: obtain image information from version agnostic structure

Make changes to the functions used for generating the DFU's alt
variable so that the FWU image information is obtained from the common
version agnostic structure instead of reading the metadata.

While here, also update the name of the field used for storing the
image GUID in the FWU metadata.

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