Add MPS compile-time option for enabling/disabling tracing

This commit adds an MPS-specific compile-time option
`MBEDTLS_MPS_TRACE` to the internal MPS header `mps/common.h`.

So far -- this may need revisiting -- MPS comes with its own
internal tracing module which allows to track the operation of
MPS' various layers for the purpose of understanding of it workings
as well as for debugging.

The reasons for the introduction of a module separate from SSL debug
are the following:

1) The SSL debug module requires an SSL context to function because
   debug callbacks are part of the runtime configuration of the SSL
   module.

   The MPS tracing module, in contrast, is not supposed to be used
   in production environments, and there is no need for a runtime
   configuration. Instead, a compile-time defined tracing callback
   is used.

2) In the interest of modularity, MPS' tracing module shouldn't
   require having an SSL context around.

3) Purely visually, MPS' tracing module adds support for indentation
   according to call-depth and coloring according to which module is
   being used, which makes it very useful for what's going on; however,
   those features aren't available in the SSL debug module (and they
   shouldn't be).

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

library/mps/common.h

@@ -47,6 +47,9 @@
  */
 #define MBEDTLS_MPS_ENABLE_ASSERTIONS
 
+/*! This flag controls whether tracing for MPS should be enabled. */
+#define MBEDTLS_MPS_TRACE
+
 /* \} name SECTION: MPS Configuration */
 
 #endif /* MBEDTLS_MPS_COMMON_H */