mbedtls: Use `tf_snprintf` instead of `snprintf`

The Trusted Firmware uses a subset of the APIs provided by mbed TLS.
This subset internally uses `snprintf`, but the only format specifier
used is '%d', which is supported by `tf_snprintf`.

This patch makes mbed TLS use `tf_snprintf` instead of `snprintf`,
saving 3 KB in both debug and release builds when TBBR is enabled.

Change-Id: I7f992a21015930d7c0f4660e7a28ceefd60b9597
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

drivers/auth/mbedtls/mbedtls_common.c

@@ -4,10 +4,11 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
-#include <assert.h>
+#include <debug.h>
 
 /* mbed TLS headers */
 #include <mbedtls/memory_buffer_alloc.h>
+#include <mbedtls/platform.h>
 
 /*
  * mbed TLS heap
@@ -29,6 +30,10 @@ void mbedtls_init(void)
 	if (!ready) {
 		/* Initialize the mbed TLS heap */
 		mbedtls_memory_buffer_alloc_init(heap, MBEDTLS_HEAP_SIZE);
+
+		/* Use reduced version of snprintf to save space. */
+		mbedtls_platform_set_snprintf(tf_snprintf);
+
 		ready = 1;
 	}
 }

include/drivers/auth/mbedtls/mbedtls_config.h

@@ -19,6 +19,8 @@
 
 #define MBEDTLS_PLATFORM_MEMORY
 #define MBEDTLS_PLATFORM_NO_STD_FUNCTIONS
+/* Prevent mbed TLS from using snprintf so that it can use tf_snprintf. */
+#define MBEDTLS_PLATFORM_SNPRINTF_ALT
 
 #define MBEDTLS_PKCS1_V15
 #define MBEDTLS_PKCS1_V21