Build: Make t_cose and QCBOR optional

To avoid errors when not building with the initial attestation partition
enabled, as optional targets are only compiled when they are a
dependency of a non-optional target.

Change-Id: If109ec1b2208c7d9138355f9e346ec1b1a3eb13f
Signed-off-by: Raef Coles <raef.coles@arm.com>