host_key2: Add VB2_SIG_ALG_COUNT to count the number of valid signatures

More reliable than simply assuming that VB2_SIG_RSA8192 is the last
signature.

BRANCH=none
BUG=chromium:684354
TEST=rm tests/testkeys/key_*; make genkeys -j
TEST=make runtests -j

Change-Id: I755b3afb50313fcdf292fb3cd5b0dfe09f8593e3
Reviewed-on: https://chromium-review.googlesource.com/438948
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
2 files changed