Add check for multiple config files being defined

Add check to check_config.h to ensure MBEDTLS_USER_CONFIG_FILE cannot be
defined if MBEDTLS_CONFIG_FILE is defined. If both are defined
MBEDTLS_USER_CONFIG_FILE will not be included, which may confuse users.
diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index fa7110f..301b283 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -30,6 +30,10 @@
+#error "MBEDTLS_USER_CONFIG_FILE cannot be included if MBEDTLS_CONFIG_FILE is defined."
  * We assume CHAR_BIT is 8 in many places. In practice, this is true on our
  * target platforms, so not an issue, but let's just be extra sure.