Attest: Get option flags from challenge object

Special option flags (for test purpose) can be passed in for
attestation service. These flags are encoded in the challenge object.
Previously a 36 bytes long challenge object was used to carry these
4 extra bytes, but attestation API requires the challenge object to be
only 32, 48 or 64 bytes long.

With this change the option flags are packed in a 64 bytes long
challenge object which has a predefined value. If the challenge object
is 64 byte long and the bytes from 4 to 63 have 0 value in this case
the first 4 bytes are handled as the option flags.

Change-Id: I2a4d0bf417905d74cca0fbdb5070cdef085fe308
Signed-off-by: Laurence Lundblade <lgl@securitytheory.com>
Co-authored-by: Tamas Ban <tamas.ban@arm.com>