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).
024a4b1f2a