Adapt buffer size for minimal CCM config

This is useful for testing interop with GnuTLS, which sends records larger
than 512 bytes.

This change is triggered by the addition of CCM interop testing with GnuTLS a
few commits ago.

configs/config-ccm-psk-tls1_2.h

@@ -81,7 +81,7 @@
  * both ends of the connection!  (See comments in "mbedtls/ssl.h".)
  * The optimal size here depends on the typical size of records.
  */
-#define MBEDTLS_SSL_MAX_CONTENT_LEN             512
+#define MBEDTLS_SSL_MAX_CONTENT_LEN             1024
 
 #include "mbedtls/check_config.h"