swtpm_cert: Add support for serial numbers up to 20 bytes long

x509 certificate serial numbers can be up to 20 bytes long.
Support this via gmp library.

A serial number must not have its most significant bit set, which
would indicate a negative number. If this is the case, insert '0'
as the first byte.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
2 files changed