Add MPS trace module implementation

This commit adds an implementation of the MPS trace module
based on `printf()`.

The enabling macro MBEDTLS_MPS_TRACE remains unset by default
because MPS tracing is very verbose and consumes unnecessary
space in the CI.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/library/Makefile b/library/Makefile
index a67160c..0fb6eeb 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -105,6 +105,7 @@
 	     md5.o \
 	     memory_buffer_alloc.o \
              mps/reader.o \
+             mps/trace.o \
 	     nist_kw.o \
 	     oid.o \
 	     padlock.o \