bdb: Add secret deriving code for SP-RO

This patch adds code which dervies secrets from BDS. It's supposed to be
done by SP-RO, hence the code is mostly useful for testing (or emulation).

vba_extend_secrets_ro takes a function pointer to a hash extend
function. It'll be used to try different sha256 extend algorithms.

BUG=chromium:649555
BRANCH=none
TEST=make runtests

Change-Id: I8fef6b851fb84686d8bcdd948b36160016687c51
Signed-off-by: Daisuke Nojiri <dnojiri@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/384354
Reviewed-by: Randall Spangler <rspangler@chromium.org>