tbbr: Add build flag HASH_ALG to let the user to select the SHA

The flag support the following values:
    - sha256 (default)
    - sha384
    - sha512

Change-Id: I7a49d858c361e993949cf6ada0a86575c3291066
Signed-off-by: Qixiang Xu <qixiang.xu@arm.com>
6 files changed