base-files: validate metadata of sysupgrade images

Use fwtool to extract it, only require metadata to be present if the
platform sysupgrade script sets REQUIRE_IMAGE_METADATA=1

Image metadata is in JSON format and contains a list of supported
devices, along with version information that could be displayed by a UI
later before the actual upgrade happens.

Signed-off-by: Felix Fietkau <nbd@nbd.name>