X.509: Move certificate length retrieval into new helper

The upcoming in-kernel SPDM library (Security Protocol and Data Model,
https://www.dmtf.org/dsp/DSP0274) needs to retrieve the length from
ASN.1 DER-encoded X.509 certificates.

Such code already exists in x509_load_certificate_list(), so move it
into a new helper for reuse by SPDM.

Export the helper so that SPDM can be tristate.  (Some upcoming users of
the SPDM libray may be modular, such as SCSI and ATA.)

No functional change intended.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
2 files changed