Create a library file for libmbedtls

TF Makefile was linking all the objects files generated for the
Mbed TLS library instead of creating a static library that could be
used in the linking stage.

Change-Id: I8e4cd843ef56033c9d3faeee71601d110b7e4c12
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>
diff --git a/drivers/auth/mbedtls/mbedtls_x509.mk b/drivers/auth/mbedtls/mbedtls_x509.mk
index a6f72e6..a0557e2 100644
--- a/drivers/auth/mbedtls/mbedtls_x509.mk
+++ b/drivers/auth/mbedtls/mbedtls_x509.mk
@@ -6,8 +6,4 @@
 
 include drivers/auth/mbedtls/mbedtls_common.mk
 
-MBEDTLS_X509_SOURCES	:=	drivers/auth/mbedtls/mbedtls_x509_parser.c	\
-				$(addprefix ${MBEDTLS_DIR}/library/,		\
-				x509.c 						\
-				x509_crt.c 					\
-				)
+MBEDTLS_SOURCES	+=	drivers/auth/mbedtls/mbedtls_x509_parser.c