Convert public key to PEM without phpseclib

Since the public key parameters and length are fixed, we don't need
a full fledged ASN.1 encoder class, and can hardcode those.

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Use OpenSSL for attestion certificate validation

Removes dependency on phpseclib

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>