SSL key export interface change
-------------------------------

This affects users of the SSL key export APIs:
```
    mbedtls_ssl_conf_export_keys_cb()
    mbedtls_ssl_conf_export_keys_ext_cb()
```

The API `mbedtls_ssl_conf_export_keys_ext_cb()` has been removed,
and the function type of key export callback passed to
`mbedtls_ssl_conf_export_keys_cb()` has changed, as follows:
- It no longer exports raw keys and IV.
- A secret type parameter has been added to identify which key
  is being exported. For TLS 1.2, only the master secret is
  exported, but upcoming TLS 1.3 support will add other kinds of keys.

For users which do not rely on raw keys and IV, adjusting to the new
callback type should be straightforward - see the example programs
programs/ssl/ssl_client2 and programs/ssl/ssl_server2 for callbacks
for NSSKeylog, EAP-TLS and DTLS-SRTP.

Users which require access to the raw keys used to secure application
traffic may derive those by hand based on the master secret and the
handshake transcript hashes which can be obtained from the raw data
on the wire. Such users are also encouraged to reach out to the
Mbed TLS team on the mailing list, to let the team know about their
use case.

Add migration guide for modified key export API
Add migration guide for modified key export API

Signed-off-by: Hanno Becker <4d488caff78d6184f59bbe6e137e34d4606b9fe8@arm.com>
