Add a utils.h file that contains common functions

The new header contains common information across various mbed TLS
modules and avoids code duplication. To start, utils.h currently only
contains the mbedtls_zeroize() function.
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index 7742c22..24a2484 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -57,6 +57,7 @@
     version.c
     version_features.c
     xtea.c
+    utils.c
 )
 
 set(src_x509