DPE: Update CertifyKey to newer spec (v 0.9)

* Returns leaf certificate only for current layer instead of full chain.
* Does not finalises the layer while returning the leaf certificate.

Signed-off-by: Maulik Patel <maulik.patel@arm.com>
Change-Id: I7aa5686564cb6b291d751053684df5f9b13f3862
5 files changed