extmod/modtls_mbedtls: Implement cert verification callback for mbedtls.

This is a useful alternative to .getpeercert() when the certificate is not
stored to reduce RAM usage.

Signed-off-by: Felix Dörre <felix@dogcraft.de>

tests/net_hosted/ssl_verify_callback.py

@@ -0,0 +1,37 @@
+# test ssl verify_callback
+
+import io
+import socket
+import tls
+
+
+def verify_callback(cert, depth):
+    print("verify_callback:", type(cert), len(cert) > 100, depth)
+    return 0
+
+
+def verify_callback_fail(cert, depth):
+    print("verify_callback_fail:", type(cert), len(cert) > 100, depth)
+    return 1
+
+
+def test(peer_addr):
+    context = tls.SSLContext(tls.PROTOCOL_TLS_CLIENT)
+    context.verify_mode = tls.CERT_OPTIONAL
+    context.verify_callback = verify_callback
+    s = socket.socket()
+    s.connect(peer_addr)
+    s = context.wrap_socket(s)
+    s.close()
+
+    context.verify_callback = verify_callback_fail
+    s = socket.socket()
+    s.connect(peer_addr)
+    try:
+        s = context.wrap_socket(s)
+    except OSError as e:
+        print(e.args[1])
+
+
+if __name__ == "__main__":
+    test(socket.getaddrinfo("micropython.org", 443)[0][-1])