Add DER-encoded CRTs to library/certs.c

[hanno-becker]

When building with MBEDTLS_PEM_PARSE_C and MBEDTLS_FS_IO unset, ssl_client2 and ssl_server2 fail to parse any hardcoded test CRTs because those are stored in PEM format in library/certs.c. This hardens testing of such minimal configurations.

A potential solution is to also provide DER-encodings of the certificates in library/certs.c.

[RonEld]

I tend to think this is a bug, because in these conditions, I would expect the application to have a stub main() functions, like when other configurations are unset.

[hanno-becker]

@RonEld Yes, I agree. Thanks for taking a look!

[ciarmcom]

ARM Internal Ref: IOTSSL-2655