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/Makefile b/library/Makefile
index 0333815..46dce4e 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -65,7 +65,8 @@
 		ripemd160.o	rsa_internal.o	rsa.o  		\
 		sha1.o		sha256.o	sha512.o	\
 		threading.o	timing.o	version.o	\
-		version_features.o		xtea.o
+		version_features.o		xtea.o	\
+		utils.o
 
 OBJS_X509=	certs.o		pkcs11.o	x509.o		\
 		x509_create.o	x509_crl.o	x509_crt.o	\