vbutil_kernel: support exporting a keyblock file during verify

Reuses the --keyblock argument to output a keyblock if used
during Verify().

TEST=built, ran on a kernel; check if it worked for cgpt find -M :)
BUG=chromium-os:7451

Change-Id: Ibf1365dbdaeaf87442e0d12d048bc070f35662ad

Review URL: http://codereview.chromium.org/4160001
1 file changed