Refuse mbed TLS external key with non RSA certificates

Refuse mbed TLS external key with non RSA certificates

The current API that we use (mbedtls_pk_setup_rsa_alt) only allows
using RSA keys with the external API. Using an EC, mbed TLS and external
key in OpenVPN will fail very late with a rather obscure error message.

Instead fail early and provide a clear message that only RSA keys are
supported.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Steffan Karger <steffan@karger.me>
Message-Id: <20181008214123.10819-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17671.html
Signed-off-by: David Sommerseth <davids@openvpn.net>