Add stub implementation for MPS tracing API

MPS' tracing module uses four macros:

1) TRACE( type, fmt, ... )

   This acts like `printf( fmt, ... )` but also allows
   the specification of a type of trace output (comment,
   warning, error, ...)

2) TRACE_INIT

   This acts like TRACE() but increases the level of
   indentation. It will be used at the beginning of
   function calls.

3) RETURN( val )

   Equivalent to `return( val )` plus a decrement in the
   level of indentation. This should be used at the end of
   functions that have been started with TRACE_INIT.

4) TRACE_END

   This combines a trace output with a decrement of the
   level of indentation. It's necessary prior to leaving
   functions which have been started with TRACE_INIT
   but which don't have a return value.

This commit defines those macros as no-op dummies in
`library/mps/trace.h` for now.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>

library/mps/common.h

@@ -48,7 +48,7 @@
 #define MBEDTLS_MPS_ENABLE_ASSERTIONS
 
 /*! This flag controls whether tracing for MPS should be enabled. */
-#define MBEDTLS_MPS_TRACE
+//#define MBEDTLS_MPS_TRACE
 
 /* \} name SECTION: MPS Configuration */