mbedtls: don't use API deprecated in mbed 2.7

mbedtls: don't use API deprecated in mbed 2.7

The void-returning mbedtls_sha256() was deprecated in mbed TLS 2.7.
Use our own md_full() abstraction instead.

(The new function can theoretically fail, but only in case of highly
unlikely digest function failures.  The personalisation on random using
the certificate is a best-effort measure, so we simply log a warning and
skip the personalisation if such highly unlikely errors occur.)

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <1518006166-14285-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16445.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit f22e89bd2311d3cab511e574746c6f82f1fa1a54)