Use negated option for controlling TLS support.

A positive option looks better, but comes with the following compatibility
issue: people using a custom config.h that is not based on the default
config.h and need TLS support would need to manually change their config in
order to still get TLS.

Work around that by making the public option negative. Internally the positive
option is used, though.

In the future (when preparing the next major version), we might want to switch
back to a positive option as this would be more consistent with other options
we have.
diff --git a/configs/config-ccm-psk-tls1_2.h b/configs/config-ccm-psk-tls1_2.h
index bd2c1a3..c9b58dd 100644
--- a/configs/config-ccm-psk-tls1_2.h
+++ b/configs/config-ccm-psk-tls1_2.h
@@ -41,7 +41,6 @@
 /* mbed TLS feature support */
 #define MBEDTLS_KEY_EXCHANGE_PSK_ENABLED
 #define MBEDTLS_SSL_PROTO_TLS1_2
-#define MBEDTLS_SSL_PROTO_TLS
 
 /* mbed TLS modules */
 #define MBEDTLS_AES_C
diff --git a/configs/config-mini-tls1_1.h b/configs/config-mini-tls1_1.h
index 349ea8e..013bc03 100644
--- a/configs/config-mini-tls1_1.h
+++ b/configs/config-mini-tls1_1.h
@@ -40,7 +40,6 @@
 #define MBEDTLS_PKCS1_V15
 #define MBEDTLS_KEY_EXCHANGE_RSA_ENABLED
 #define MBEDTLS_SSL_PROTO_TLS1_1
-#define MBEDTLS_SSL_PROTO_TLS
 
 /* mbed TLS modules */
 #define MBEDTLS_AES_C
diff --git a/configs/config-suite-b.h b/configs/config-suite-b.h
index e6fad1c..18e2c40 100644
--- a/configs/config-suite-b.h
+++ b/configs/config-suite-b.h
@@ -47,7 +47,6 @@
 #define MBEDTLS_ECP_DP_SECP384R1_ENABLED
 #define MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED
 #define MBEDTLS_SSL_PROTO_TLS1_2
-#define MBEDTLS_SSL_PROTO_TLS
 
 /* mbed TLS modules */
 #define MBEDTLS_AES_C
diff --git a/configs/config-thread.h b/configs/config-thread.h
index 3166aa9..4fa0b8d 100644
--- a/configs/config-thread.h
+++ b/configs/config-thread.h
@@ -49,6 +49,7 @@
 #define MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
 #define MBEDTLS_SSL_PROTO_TLS1_2
 #define MBEDTLS_SSL_PROTO_DTLS
+#define MBEDTLS_SSL_PROTO_NO_TLS
 #define MBEDTLS_SSL_DTLS_ANTI_REPLAY
 #define MBEDTLS_SSL_DTLS_HELLO_VERIFY
 #define MBEDTLS_SSL_EXPORT_KEYS