extmod/modtls_mbedtls: Wire in support for DTLS.

This commit enables support for DTLS, i.e. TLS over datagram transport
protocols like UDP.  While support for DTLS is absent in CPython, it is
worth supporting it in MicroPython because it is the basis of the
ubiquitous CoAP protocol, used in many IoT projects.

To select DTLS, a new set of "protocols" are added to SSLContext:
- ssl.PROTOCOL_DTLS_CLIENT
- ssl.PROTOCOL_DTLS_SERVER

If one of these is set, the library assumes that the underlying socket is a
datagram-like socket (i.e. UDP or similar).

Our own timer callbacks are implemented because the out of the box
implementation relies on `gettimeofday()`.

This new DTLS feature is enabled on all ports that use mbedTLS.

This commit is an update to a previous PR #10062.

Addresses issue #5270 which requested DTLS support.

Signed-off-by: Keenan Johnson <keenan.johnson@gmail.com>

extmod/mbedtls/mbedtls_config_common.h

@@ -89,6 +89,7 @@
 #define MBEDTLS_SHA384_C
 #define MBEDTLS_SHA512_C
 #define MBEDTLS_SSL_CLI_C
+#define MBEDTLS_SSL_PROTO_DTLS
 #define MBEDTLS_SSL_SRV_C
 #define MBEDTLS_SSL_TLS_C
 #define MBEDTLS_X509_CRT_PARSE_C