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/CMakeLists.txt b/library/CMakeLists.txt
index 67074d6..2c1bccb 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -48,6 +48,7 @@
     md5.c
     memory_buffer_alloc.c
     mps/reader.c
+    mps/trace.o
     nist_kw.c
     oid.c
     padlock.c