generate default tls cert+key
If no RDP key or TLS key is available in the config or on the commandline, generate a default one for the session using OpenSSL. If the OpenSSL package is not available at build time, maintain the existing fail behavior.
This lets users do weston --backend=rdp-backend.so
without any further arguments. I did look at using winpr/tools/makecert.h
but the public-facing api doesn't appear to be rich enough to generate a string buffer for peer->settings->CertificateContent
.