Add PoC of Arm DRTM specification version Beta-0

Change-Id: I26e6f2d4b2299edc246f5e8504d5d15b1399f640
diff --git a/drivers/auth/mbedtls/mbedtls_common.mk b/drivers/auth/mbedtls/mbedtls_common.mk
index 53ebe30..ace4312 100644
--- a/drivers/auth/mbedtls/mbedtls_common.mk
+++ b/drivers/auth/mbedtls/mbedtls_common.mk
@@ -16,7 +16,7 @@
 MBEDTLS_INC		=	-I${MBEDTLS_DIR}/include
 
 # Specify mbed TLS configuration file
-MBEDTLS_CONFIG_FILE	:=	"<drivers/auth/mbedtls/mbedtls_config.h>"
+MBEDTLS_CONFIG_FILE	?=	"<drivers/auth/mbedtls/mbedtls_config.h>"
 $(eval $(call add_define,MBEDTLS_CONFIG_FILE))
 
 MBEDTLS_SOURCES	+=		drivers/auth/mbedtls/mbedtls_common.c