altcp_tls_mbedtls: convert #error on too small TCP_WND to warning

Many TLS use cases are OK with a small TCP_WND, so don't prevent these
by having a preprocessor check that cannot be disabled.

src/apps/altcp_tls/altcp_tls_mbedtls.c

@@ -85,10 +85,6 @@
 
 #include <string.h>
 
-#if TCP_WND < MBEDTLS_SSL_MAX_CONTENT_LEN
-#error TCP_WND < MBEDTLS_SSL_MAX_CONTENT_LEN, cannot receive a full decryption buffer
-#endif
-
 #ifndef ALTCP_MBEDTLS_ENTROPY_PTR
 #define ALTCP_MBEDTLS_ENTROPY_PTR   NULL
 #endif
@@ -675,6 +671,11 @@ altcp_tls_create_config(int is_server, int have_cert, int have_pkey, int have_ca
   struct altcp_tls_config *conf;
   mbedtls_x509_crt *mem;
 
+  if (TCP_WND < MBEDTLS_SSL_MAX_CONTENT_LEN) {
+    LWIP_DEBUGF(ALTCP_MBEDTLS_DEBUG|LWIP_DBG_LEVEL_SERIOUS,
+      ("altcp_tls: TCP_WND is smaller than the RX decrypion buffer, connection RX might stall!\n"));
+  }
+
   altcp_mbedtls_mem_init();
 
   sz = sizeof(struct altcp_tls_config);