Fix the maximum output size in the call to the external signer.

Maximum output size is the signature size.

BUG=7676
TEST=manual

1) Verified that earlier outbufsize value was more than what the external signer would return.
2) Re-ran run_vbutil_tests.sh

Change-Id: I180cfea7625ee09a51709d8f7735884c32b8b409

Review URL: http://codereview.chromium.org/4251006
1 file changed