refactor(mbedtls): allow platform to specify their config file

Common mbedTLS implementation include the fixed configuration
file of mbedTLS and that does not gives flexilibility to the
platform to include their own mbedTLS configuration.
Hence changes are done so that platform can include their own
mbedTLS configuration file.

Signed-off-by: Lucian Paul-Trifu <lucian.paul-trifu@arm.com>
Signed-off-by: Manish V Badarkhe <manish.badarkhe@arm.com>
Change-Id: I04546589f67299e26b0a6a6e151cdf1fdb302607
diff --git a/drivers/auth/mbedtls/mbedtls_common.mk b/drivers/auth/mbedtls/mbedtls_common.mk
index 54c819c..0a4775d 100644
--- a/drivers/auth/mbedtls/mbedtls_common.mk
+++ b/drivers/auth/mbedtls/mbedtls_common.mk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2015-2021, Arm Limited. All rights reserved.
+# Copyright (c) 2015-2022, Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -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