A positive option looks better, but comes with the following compatibility
issue: people using a custom config.h that is not based on the default
config.h and need TLS support would need to manually change their config in
order to still get TLS.
Work around that by making the public option negative. Internally the positive
option is used, though.
In the future (when preparing the next major version), we might want to switch
back to a positive option as this would be more consistent with other options
we have.
For now the option has no effect.
Adapted existing example config files. The fact that I needed to do this
highlights that this is a slightly incompatible change: existing users need to
update their existing custom configs (if standalone as opposed to based on the
default config) in order to still get the same behaviour.
The alternative would be to have a negative config option (eg NO_TLS or
DTLS_ONLY) but this doesn't fit as nicely with the existing options, so
hopefully the minor incompatibility is acceptable.
I don't think it's worth adding a new component to all.sh:
- builds with both DTLS and TLS are done in the default (and full) config
- TLS-only builds are done with eg config-suite-b.h in test-ref-configs
- a DTLS-only build is done with config-thread.h in test-ref-configs
- builds with none of them (and SSL_TLS_C enabled) are forbidden
