Cleanup ParseCertRelative code

Fix for case:
- can't find a signer for a certificate with the AKID
- find it by name
Has to error as the signer's SKID is always set for signer and would
have matched the AKID.
Simplify the path length code - don't look up CA twice.
Don't require the tsip_encRsaKeyIdx field in DecodedCert when
!WOLFSSL_RENESAS_TSIP - use local variable.