stm32/mbedtls: Enable MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_KEY_EXCHANGE.

This adds MBEDTLS_MD_SHA1 to the list of default hashes for TLS 1.2
handshake signatures.  Although SHA-1 is weak, this option is turned on in
the default mbedtls configuration file, and allows better compatibility
with older servers.  In particular it allows an stm32-mbedtls-based client
to connect to an axtls-based client (eg default unix port and esp8266).

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

ports/stm32/mbedtls/mbedtls_config.h

@@ -82,6 +82,7 @@
 #define MBEDTLS_SSL_CLI_C
 #define MBEDTLS_SSL_SRV_C
 #define MBEDTLS_SSL_TLS_C
+#define MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_KEY_EXCHANGE
 #define MBEDTLS_X509_CRT_PARSE_C
 #define MBEDTLS_X509_USE_C