extmod/modussl_mbedtls: Add SSLContext.set_cert_key() method.
extmod/modussl_mbedtls: Add SSLContext.set_cert_key() method.

Allows to set own certificate and key (both in DER format, passed as
binary buffers).

The closest Python "ssl" API has is SSLContext.load_cert_chain(), but
that deals with "files", supports non-binary encodings like PEM,
additional feature like encrypted keys, etc. "ussl" module provides
just a baseline API, hence a specific method like set_cert_key(),
on top of which compatibility APIs like load_cert_chain() can be
implemented.

Change-Id: Idfe0248d26363ae06bf731931f593ac2b5100ac7
Signed-off-by: Paul Sokolovsky <9dc5061f178bb11813e14f1ed02ba27eaf21fa5d@users.sourceforge.net>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython