Fix and simplify *-PSK ifdef's
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index e254f3a..7450d34 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -825,10 +825,7 @@
 #endif /* POLARSSL_SHA512_C */
 #endif /* POLARSSL_SSL_PROTO_TLS1_2 */
 
-#if defined(POLARSSL_KEY_EXCHANGE_PSK_ENABLED) ||                           \
-    defined(POLARSSL_KEY_EXCHANGE_RSA_PSK_ENABLED) ||                       \
-    defined(POLARSSL_KEY_EXCHANGE_DHE_PSK_ENABLED) ||                       \
-    defined(POLARSSL_KEY_EXCHANGE_ECDHE_PSK_ENABLED)
+#if defined(POLARSSL_KEY_EXCHANGE__SOME__PSK_ENABLED)
 int ssl_psk_derive_premaster( ssl_context *ssl, key_exchange_type_t key_ex )
 {
     unsigned char *p = ssl->handshake->premaster;
@@ -926,10 +923,7 @@
 
     return( 0 );
 }
-#endif /* POLARSSL_KEY_EXCHANGE_PSK_ENABLED ||
-          POLARSSL_KEY_EXCHANGE_RSA_PSK_ENABLED ||
-          POLARSSL_KEY_EXCHANGE_DHE_PSK_ENABLED ||
-          POLARSSL_KEY_EXCHANGE_ECDHE_PSK_ENABLED */
+#endif /* POLARSSL_KEY_EXCHANGE__SOME__PSK_ENABLED */
 
 #if defined(POLARSSL_SSL_PROTO_SSL3)
 /*
@@ -3675,10 +3669,7 @@
 }
 #endif /* POLARSSL_X509_CRT_PARSE_C */
 
-#if defined(POLARSSL_KEY_EXCHANGE_PSK_ENABLED) ||                           \
-    defined(POLARSSL_KEY_EXCHANGE_RSA_PSK_ENABLED) ||                       \
-    defined(POLARSSL_KEY_EXCHANGE_DHE_PSK_ENABLED) ||                       \
-    defined(POLARSSL_KEY_EXCHANGE_ECDHE_PSK_ENABLED)
+#if defined(POLARSSL_KEY_EXCHANGE__SOME__PSK_ENABLED)
 int ssl_set_psk( ssl_context *ssl, const unsigned char *psk, size_t psk_len,
                  const unsigned char *psk_identity, size_t psk_identity_len )
 {
@@ -3714,10 +3705,7 @@
     ssl->f_psk = f_psk;
     ssl->p_psk = p_psk;
 }
-#endif /* POLARSSL_KEY_EXCHANGE_PSK_ENABLED ||
-          POLARSSL_KEY_EXCHANGE_RSA_PSK_ENABLED ||
-          POLARSSL_KEY_EXCHANGE_DHE_PSK_ENABLED ||
-          POLARSSL_KEY_EXCHANGE_ECDHE_PSK_ENABLED */
+#endif /* POLARSSL_KEY_EXCHANGE__SOME__PSK_ENABLED */
 
 #if defined(POLARSSL_DHM_C)
 int ssl_set_dh_param( ssl_context *ssl, const char *dhm_P, const char *dhm_G )
@@ -4385,9 +4373,7 @@
     }
 #endif
 
-#if defined(POLARSSL_KEY_EXCHANGE_PSK_ENABLED) ||                           \
-    defined(POLARSSL_KEY_EXCHANGE_DHE_PSK_ENABLED) ||                       \
-    defined(POLARSSL_KEY_EXCHANGE_ECDHE_PSK_ENABLED)
+#if defined(POLARSSL_KEY_EXCHANGE__SOME__PSK_ENABLED)
     if( ssl->psk != NULL )
     {
         memset( ssl->psk, 0, ssl->psk_len );