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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>