Keep module compatibility with mbedtls 2.x (old LTS branch).
A patch has been added to allow compiling after removing all the `psa_*`
files from the library folder (will look into upstreaming it).
Note: mbedTLS 3.6 finally enabled TLSv1.3 by default, but it requires
some module changes, and to enable PSA crypto (new "standard" API
specification), so it might be best done in a separate commit/PR.
This commit adds a new mbedTLS configuration header to customize the
built-in library (and can be optionally replaced by a platform-specific
one).
Currently, it disables most weak cryptographic functions (with the
notable exceptions of MD5 and SHA-1), along with removing support for
TLS versions 1.0 and 1.1 (making TLSv1.2 the only supported one).
