Attest: Calculate Instance ID only once

Instance ID is computed every time when get_token API is called.
Modify this behaviour to compute only at the first time when get_token
API is called and store it in a static buffer.
Next time just read it from static buffer and include it to the token.

Signed-off-by: Balint Matyi <Balint.Matyi@arm.com>
Change-Id: I2f9ccb70aba5cf401fcd7c5c7fa7a9d358283631
2 files changed