mbedtls: support configuration of PSK maximum length via Kconfig

So far maximum PSK length was configured by mbedTLS as 32 bytes.
Introduce Kconfig option that will configure it instead of relying on
default value from mbedTLS library, so that user can easily adjust that
value to application needs.

Signed-off-by: Marcin Niestroj <[email protected]>

Author: mniestroj

modules/mbedtls/Kconfig.tls-generic

@@ -73,6 +73,20 @@ config MBEDTLS_KEY_EXCHANGE_ECDHE_PSK_ENABLED
 config MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED
 	bool "Enable the RSA-PSK based ciphersuite modes"
 
+config MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED
+	bool
+	default y if MBEDTLS_KEY_EXCHANGE_PSK_ENABLED || \
+		MBEDTLS_KEY_EXCHANGE_DHE_PSK_ENABLED || \
+		MBEDTLS_KEY_EXCHANGE_ECDHE_PSK_ENABLED || \
+		MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED
+
+config MBEDTLS_PSK_MAX_LEN
+	int "Max size of TLS pre-shared keys"
+	default 32
+	depends on MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED
+	help
+	  Max size of TLS pre-shared keys, in bytes.
+
 config MBEDTLS_KEY_EXCHANGE_RSA_ENABLED
 	bool "Enable the RSA-only based ciphersuite modes"
 	default y if !NET_L2_OPENTHREAD

modules/mbedtls/configs/config-tls-generic.h

@@ -86,6 +86,10 @@
 #define MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED
 #endif
 
+#if defined(CONFIG_MBEDTLS_PSK_MAX_LEN)
+#define MBEDTLS_PSK_MAX_LEN	CONFIG_MBEDTLS_PSK_MAX_LEN
+#endif
+
 #if defined(CONFIG_MBEDTLS_KEY_EXCHANGE_RSA_ENABLED)
 #define MBEDTLS_KEY_EXCHANGE_RSA_ENABLED
 #endif