COSE: Split t_cose to two part: sign and verify

Increase granularity to link only those files to secure
code which are used for signing the attestation token.
Other files are linked to non-secure code which are used
during attestation test suite for signature verification.

Change-Id: I6dc030563f56b047253c7a1f3494f59a12fe61c7
Signed-off-by: Tamas Ban <tamas.ban@arm.com>