Interface: TF-M specific log API

Provide a TF-M specific log API. Put the source in 'interface' folder
since this file are referenced by multiple components, such as bl2,
non-secure application and secure firmware. The sources are included
into SPRTL building and as part of SPRTL.
The fundamental API is 'tfm_log_printf' which supports basic formats
like 'sdicpuxX' and '%' as the format leading and escape.

Change-Id: I6d0f89ab16c3e00f8e71129a06f89dc206733733
Signed-off-by: Mingyang Sun <mingyang.sun@arm.com>
diff --git a/platform/ext/common/uart_stdout.h b/platform/ext/common/uart_stdout.h
index 8feff31..d967c7c 100644
--- a/platform/ext/common/uart_stdout.h
+++ b/platform/ext/common/uart_stdout.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2018 ARM Limited
+ * Copyright (c) 2017-2019 ARM Limited
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -40,4 +40,9 @@
  */
 void stdio_uninit(void);
 
+/**
+ * \brief Output buffer by STDIO.
+ */
+int stdio_output_string(const unsigned char *str, uint32_t len);
+
 #endif /* __UART_STDOUT_H__ */