extmod/modssl_mbedtls: Make SSLSocket.getpeercert() optional.

And only enable this method when the relevant feature is available in
mbedtls.  Otherwise, if mbedtls doesn't support getting the peer
certificate, this method always returns None and it's confusing why it does
that.  It's better to remove the method altogether, so the error trying to
use it is more obvious.

Signed-off-by: Damien George <damien@micropython.org>

tests/net_hosted/ssl_getpeercert.py

@@ -1,8 +1,15 @@
 # test ssl.getpeercert() method
 
+import io
 import socket
 import ssl
 
+s_test = ssl.wrap_socket(io.BytesIO(), server_side=True, do_handshake=False)
+s_test.close()
+if not hasattr(s_test, "getpeercert"):
+    print("SKIP")
+    raise SystemExit
+
 
 def test(peer_addr):
     s = socket.socket()