feat(fwu): pass a const metadata structure to platform routines

The metadata structure copy is passed to the platform routine to set
the image source to boot the platform from. This is done by reading
the metadata structure. Pass the metadata as a read-only copy to the
routine -- the routine only needs to consume the metadata values and
should not be able to update the metadata fields.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Change-Id: I399cad99ab89c71483e5a32a1de0e22df304f8b0