Add setbuf platform function

Add a platform function mbedtls_setbuf(), defaulting to setbuf().

The intent is to allow disabling stdio buffering when reading or writing
files with sensitive data, because this exposes the sensitive data to a
subsequent memory disclosure vulnerability.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 1ced6e5..0ac9cca 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -368,6 +368,20 @@
 #error "MBEDTLS_PLATFORM_EXIT_MACRO and MBEDTLS_PLATFORM_STD_EXIT/MBEDTLS_PLATFORM_EXIT_ALT cannot be defined simultaneously"
 #endif
 
+#if defined(MBEDTLS_PLATFORM_SETBUF_ALT) && !defined(MBEDTLS_PLATFORM_C)
+#error "MBEDTLS_PLATFORM_SETBUF_ALT defined, but not all prerequisites"
+#endif
+
+#if defined(MBEDTLS_PLATFORM_SETBUF_MACRO) && !defined(MBEDTLS_PLATFORM_C)
+#error "MBEDTLS_PLATFORM_SETBUF_MACRO defined, but not all prerequisites"
+#endif
+
+#if defined(MBEDTLS_PLATFORM_SETBUF_MACRO) &&\
+    ( defined(MBEDTLS_PLATFORM_STD_SETBUF) ||\
+        defined(MBEDTLS_PLATFORM_SETBUF_ALT) )
+#error "MBEDTLS_PLATFORM_SETBUF_MACRO and MBEDTLS_PLATFORM_STD_SETBUF/MBEDTLS_PLATFORM_SETBUF_ALT cannot be defined simultaneously"
+#endif
+
 #if defined(MBEDTLS_PLATFORM_TIME_ALT) &&\
     ( !defined(MBEDTLS_PLATFORM_C) ||\
         !defined(MBEDTLS_HAVE_TIME) )