TF-A: Add HASH_ALG default value to defaults.mk

This patch adds default value of 'sha256' for HASH_ALG
build flag to 'make_helpers\defaults.mk', according to
'docs\getting_started\build-options.rst'.
This fixes Measured Boot driver error when TF-A uses
default HASH_ALG value and TPM_HASH_ALG is set to
sha384 or sha512.

Change-Id: Id0aa34b54807de0adaf88e5f7d7032577c22f365
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>

make_helpers/defaults.mk

 #
-# Copyright (c) 2016-2020, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2016-2020, ARM Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -150,6 +150,10 @@ GICV2_G0_FOR_EL3		:= 0
 # by lower ELs.
 HANDLE_EA_EL3_FIRST		:= 0
 
+# Secure hash algorithm flag, accepts 3 values: sha256, sha384 and sha512.
+# The default value is sha256.
+HASH_ALG			:= sha256
+
 # Whether system coherency is managed in hardware, without explicit software
 # operations.
 HW_ASSISTED_COHERENCY		:= 0