lwip: tls: enforce checking of server certificates based on CA availability

lwip: tls: enforce checking of server certificates based on CA availability

Instead of relying on some build time configuration to determine if
server certificates need to be checked against CA certificates, do it
based on the availability of such certificates. If no CA is configured
then no check can succeed; on the other hand if we have CA certs then
we should not ignore them. It is always possible to remove the CA certs
(via 'wget cacert 0 0') to force an HTTPS download that would fail
certificate validation.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>