efi: add a helper to generate dynamic UUIDs

efi: add a helper to generate dynamic UUIDs

Introduce a new helper efi_capsule_update_info_gen_ids() which populates
the capsule update fw images image_type_id field. This allows for
determinstic UUIDs to be used that can scale to a large number of
different boards and board variants without the need to maintain a big
list.

We call this from efi_fill_image_desc_array() to populate the UUIDs
lazily on-demand.

Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>