Fix #ifdef inconsistency

fixes #310

Actually all key exchanges that use a certificate use signatures too, and
there is no key exchange that uses signatures but no cert, so merge those two
flags.

Conflicts:
	ChangeLog
diff --git a/include/mbedtls/ssl_ciphersuites.h b/include/mbedtls/ssl_ciphersuites.h
index b322f42..55fae0b 100644
--- a/include/mbedtls/ssl_ciphersuites.h
+++ b/include/mbedtls/ssl_ciphersuites.h
@@ -246,6 +246,7 @@
     MBEDTLS_KEY_EXCHANGE_ECDH_ECDSA,
 } mbedtls_key_exchange_type_t;
 
+/* Key exchanges using a certificate */
 #if defined(MBEDTLS_KEY_EXCHANGE_RSA_ENABLED)           || \
     defined(MBEDTLS_KEY_EXCHANGE_DHE_RSA_ENABLED)       || \
     defined(MBEDTLS_KEY_EXCHANGE_ECDHE_RSA_ENABLED)     || \
@@ -256,6 +257,7 @@
 #define MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED
 #endif
 
+/* Key exchanges using a PSK */
 #if defined(MBEDTLS_KEY_EXCHANGE_PSK_ENABLED)           || \
     defined(MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED)       || \
     defined(MBEDTLS_KEY_EXCHANGE_DHE_PSK_ENABLED)       || \
@@ -263,18 +265,13 @@
 #define MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED
 #endif
 
+/* Key exchanges using a ECDHE */
 #if defined(MBEDTLS_KEY_EXCHANGE_ECDHE_RSA_ENABLED)     || \
     defined(MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED)   || \
     defined(MBEDTLS_KEY_EXCHANGE_ECDHE_PSK_ENABLED)
 #define MBEDTLS_KEY_EXCHANGE__SOME__ECDHE_ENABLED
 #endif
 
-#if defined(MBEDTLS_KEY_EXCHANGE_DHE_RSA_ENABLED)       || \
-    defined(MBEDTLS_KEY_EXCHANGE_ECDHE_RSA_ENABLED)     || \
-    defined(MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED)
-#define MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED
-#endif
-
 typedef struct mbedtls_ssl_ciphersuite_t mbedtls_ssl_ciphersuite_t;
 
 #define MBEDTLS_CIPHERSUITE_WEAK       0x01    /**< Weak ciphersuite flag  */